Watch the full interview with Ukrainian President Petro Poroshenko on CNN International at 5 p.m. ET Thursday in the United States and 8 p.m. Central European Time in Europe.
Brussels, Belgium (CNN) -- New Ukrainian President Petro Poroshenko says peace is possible if Russian President Vladimir Putin is in the right mood.
"Sometimes, the position of Mr. Putin is quite pragmatic, sometimes it is very emotional," Poroshenko told CNN's Christiane Amanpour on Thursday in an exclusive interview, his first broadcast interview since taking over as Ukraine's leader on June 7. "I just try to find out the time when he is more pragmatic than emotional."
He said negotiations with Russian separatists in eastern Ukraine would continue on Friday, the day his unilaterally declared cease-fire expires and the day that he also will sign a cooperation agreement with the European Union that sparked the crisis in his country.
The talks are intended to end fighting between the Moscow-backed separatists and Ukrainian military and militia forces that threatens to escalate into a broader conflict.
"I am optimistic and I'm thinking that within a few weeks, maybe months, we can have a deal to establish peace," Poroshenko said in the interview with Amanpour while he was in Brussels for EU talks that will include signing the cooperation agreement between Europe and the former Soviet satellite.
With the signing occurring the same day Poroshenko's seven-day cease-fire expires, concerns increased of further unrest in parts of eastern Ukraine and economic retaliation by Putin.
Asked by Amanpour if he was the person who could forge a peace deal with Putin, Poroshenko said that was his goal.
"I'm ready to make peace with anybody"
"I'm ready to make a peace deal with anybody," he said. "I want to bring the peace to my country, not because we are weak, not because we are less patriotic than anybody. We are ready to defend my country because I hate the idea not to use the last opportunity to bring the peace to the region."
Russia and Ukraine have been engaged in a tense standoff since March when Russia annexed the previously Ukrainian peninsula of Crimea and massed troops along other parts of its border with Ukraine.
In an effort to calm the situation, Poroshenko declared a cease-fire last week in Kiev's fight against pro-Russia separatists, but the violence continued.
On Thursday, a Ukrainian national guard base came under attack in Donetsk, Anti-Terrorist Operation unit spokesman Vladislav Seleznyov said. Ukrainian troops were able to repel the attack but were bracing for another assault, according to Seleznyov.
Two days earlier, Ukrainian authorities said pro-Russia militants shot down a military helicopter in eastern Ukraine, killing nine.
Poroshenko told Amanpour that Friday's negotiations, which will include European officials, must show some progress before his unilateral cease-fire expires at the end of the day. He seeks a commitment to negotiations, the release of hostages, a cease-fire and the withdrawal of Russian-backed forces from Ukraine.
The Organization for Security and Co-operation in Europe, which operates a special monitoring mission in eastern Ukraine, said Thursday that four members of a Donetsk-based team of OSCE monitors apparently held by separatists had been released.
Without Russian withdrawal, "it is a real war"
"All the troops on Ukrainian territory are Russian, they are Russian citizens," he said, adding that separatist leaders were from the Russian secret service. "If that continues, it is a real war."
Asked what would happen without a sufficient response from the other side, Poroshenko only said he would make that decision Friday.
On Wednesday, Russia's upper house of parliament voted, at Putin's request, to revoke the President's right to use troops in Ukraine. The move appeared to be an effort by Putin to defuse tension before Friday's watershed Ukraine-EU moment, which Putin opposes.
Poroshenko called signing the agreement with the EU the second-most important moment in his nation's history, ranking only behind independence.
"Geographically we are already in Europe," he said, adding that Ukraine connected with European values.
Asked whether he was worried about a negative economic reaction on Putin's part, Poroshenko said the Russian President recently promised not to undertake such steps.
"He promised that we will have a negotiation in a trilateral format, together with a European Union representative, so we do not expect any immediate negative reaction," Poroshenko said.
At the same time, he made clear that in his mind, the Russian annexation of Crimea -- which has an ethnic Russian majority -- cannot stand.
"Crimea is Ukrainian"
Calling the issue a top priority, Poroshenko declared: "Crimea is Ukrainian and the whole world confirmed that Crimea is Ukrainian."
Peace talks involving representatives from all sides occurred Wednesday in the Ukrainian city of Donetsk, Ukraine's official Ukrinform news agency said.
Those participating included Ukrainian government officials, pro-Russia separatists from the restive eastern Luhansk and Donetsk regions, Russian officials and members of the Organization for Security and Cooperation in Europe.
Meanwhile, U.S. Secretary of State John Kerry said the United States and Europe plan on possible further sanctions aimed at specific sectors of Russia's economy, depending on what choices Moscow makes ahead.
"It is critical for Russia to show in the next hours, literally, that they are moving to help disarm the separatists, to encourage them to disarm, to call on them to lay down their weapons and begin to become part of a legitimate political process," Kerry said Thursday after meeting with French Foreign Minister Laurent Fabius.
CNN's Mick Krever reported from Brussels for this report, which Tom Cohen wrote in Atlanta.