Donetsk, Ukraine (CNN) -- Six members of the Ukrainian armed forces were killed on Tuesday in a "terrorist attack," the Ukrainian Defense Ministry said in a statement on its website.
The incident took place in the village of Oktyabrski in the Slovyansk region, about 20 kilometers from Kramatorsk, during "a unit movement from the military base." The location is in volatile eastern Ukraine.
"Our soldiers were attacked in an ambush. Terrorists attacked our land troops with grenades. The attackers were more than 30 people and set an ambush near the river," the ministry said.
"After a long shootout, six soldiers of The Ukrainian Armed Services were killed," the statement said.
In another incident in eastern Ukraine, a separatist leader has been injured in a suspected assassination attempt, a spokesman said Tuesday, amid continuing turmoil in the wake of a controversial weekend referendum on independence.
A car carrying the "Luhansk people's governor" Valeriy Bolotov was fired on Tuesday in the Luhansk region, said Vasiliy Nikitin, a spokesman for the self-declared "Luhansk People's Republic."
Bolotov suffered a gunshot injury, but Nikitin said it was "light" and not life threatening. It is not known who was behind the shooting.
The reported attacks comesamid simmering tensions in Donetsk and Luhansk regions, where pro-Russian separatists staged a referendum Sunday asking residents whether they should declare independence from Ukraine.
Speaking in Brussels, Ukrainian Prime Minister Arseniy Yatsenyuk had strong words for Russia, saying it continues to support the separatists behind the unrest.
"Russia will fail to make a failed state," he said, as he urged Moscow to condemn the pro-Russian militants.
Yatsenyuk said the priority for Ukraine was to hold free and fair national elections on May 25, after which, he said, "we expect to have a new, legitimate president."
The Prime Minister warned that Ukraine would pursue Russia through the courts over its annexation in March of Ukraine's Crimea territory, including an oil and natural gas company based there.
Ukraine will also challenge Russian energy giant Gazprom in court unless it agrees to renegotiate the price it charges Ukraine for natural gas supplies, Yatsenyuk said. Gazprom said the recent sharp increase, from $268.50 to $485 per 1,000 cubic meters, was necessary because Ukraine is billions of dollars in arrears.
Yatsenyuk said Ukraine would pay what it owes, but only if Gazprom revises the natural gas deal in line with market rates.
"Russia is to stop using natural gas as another type of Russian weapon," he said.
German Foreign Minister Frank-Walter Steinmeier, speaking alongside Yatsenyuk earlier in Kiev, warned that the situation in eastern Ukraine is "still dangerous and threatening," and backed efforts by the interim government to start a national dialogue.
Steinmeier said the May 25 presidential elections would play a "decisive role" in restoring calm to Ukraine and urged steps to disarm the illegal separatist groups who have seized key buildings in the east.
He also praised the interim government for its handling of the crisis.
Steinmeier's visit is the latest in a series by foreign diplomats seeking a peaceful resolution to what has become the worst East-West crisis since the end of the Cold War.
Their efforts have done little so far to prevent pro-Russian militants from tightening their grip on Ukraine's east and south
Nearly 90% of voters in the Donetsk area favored secession, the head of the central election commission for the self-declared Donetsk People's Republic said Tuesday. He said just over 10% voted against the move.
Separatist leader Denis Pushilin said Monday that the Donetsk region was not only independent, but also would ask to join Russia. There was no immediate response from Ukraine's government or the European Union.
Pushilin's announcement was reminiscent of separatists' moves in the Black Sea peninsula of Crimea, which Russia annexed after Crimeans voted to secede from Ukraine and join Russia in a March 16 referendum.
Acting Ukrainian President Oleksandr Turchynov said Monday of the referendum: "That farce the terrorists call a referendum is nothing else but a propagandist cover for killings, kidnapping, violence and other grave crimes."
EU, Canada impose sanctions
Sunday's referendum was also widely condemned by the international community.
In its wake, Western leaders have imposed fresh sanctions in the hopes of pressuring Russia into reining in the pro-Russian militants.
The European Union sanctioned 13 people Monday over the Ukraine crisis, bringing the total number subject to EU visa bans and asset freezes to 61, an EU diplomat said. Two Crimean entities are also sanctioned.
Those targeted in the latest round, whose names were only released Tuesday, include Vyacheslav Ponomaryov, the self-declared Mayor of Slovyansk, a rebel stronghold in the Donetsk region, and Vladimir Shamanov, commander of the Russian airborne troops.
Meanwhile, Canada has imposed sanctions on 12 additional people, six Russians and six Ukrainians, Canadian Prime Minister Stephen Harper said in a statement.
Canada's expanded sanctions list includes Russian military chief Valery Gerasimov; Russian State Duma vice-speakers Sergei Neverov and Lyudmila Shevtsova; and Igor Girkin, known as "Strelok," who is accused of being a Russian saboteur and militant leader in eastern Ukraine.
Also named by Canada are Crimean politicians and five representatives of the self-proclaimed republics and militias in the Donetsk and Luhansk regions.
Canada's government had previously approved sanctions against nine Russian politicians and businessmen, two Russian credit organizations and 16 Russian companies.
Russia, which said it respected the will of the people of Luhansk and Donetsk in Sunday's vote, has not so far responded to Pushilin's announcement that he will seek annexation by the Russian Federation.
Moscow denies having direct influence over the separatist groups. They went ahead with the referendum despite a call from Russian President Vladimir Putin to delay it.
Russian Deputy Foreign Minister Sergei Ryabkov told reporters Monday that Russia finds the EU sanctions highly regrettable, according to Russian state media.
"It's an absolutely thoughtless and irresponsible policy that doesn't match reality in any way," news agency ITAR Tass quoted him as saying.
Hague: More sanctions in the pipeline
Addressing Parliament in London on Tuesday, UK Foreign Secretary William Hague urged Russia to use its influence to de-escalate the situation and disarm the militants -- or face more tough measures.
Additional sanctions are being prepared, he said, and the European Union is ready to impose them if needed.
"Because we have now widened the criteria substantially there are now many more individuals and entities who could be added," he said of the sanctions list.
Hague added that planning for a range of wider economic and trade measures is "at an advanced stage."
EU members accepted that such measures would hurt their own economies as well as Russia's, Hague said, but were united over the "triggers" that would mean they were brought into force.
On Twitter, Hague said that Britain would back Ukraine's May 25 presidential elections by providing 100 observers for the Organization for Security and Cooperation mission and £429,000 ($723,000) in financial aid.
CNN's Atika Shubert and Kellie Morgan reported from Donetsk and Laura Smith-Spark reported and wrote in London. Journalist Lena Kashkarova and CNN's Lindsay Isaac contributed to this report.