(CNN) -- World powers expressed alarm Wednesday about a critical report documenting Iran's progress toward making a nuclear bomb, with the British foreign secretary warning that his nation would take strong measures if the Islamic republic did not change direction.
In its most detailed report to date on the military dimensions of Iran's nuclear program, the International Atomic Energy Agency (IAEA) found no evidence that Iran has decided to actually build a bomb. But its nuclear program is more ambitious and structured, and more progress has been made than previously known, the report said.
"The agency has serious concerns regarding possible military dimensions to Iran's nuclear program," the report said. "After assessing carefully and critically the extensive information available to it, the agency finds the information to be, overall, credible. The information indicates that Iran has carried out activities relevant to the development of a nuclear explosive device."
British Foreign Secretary William Hague said Britain has extended a hand of reconciliation to Iran again and again. It is time, he said, for Iran to accept.
"We are prepared to have further talks, but only if Iran is prepared to engage in serious negotiations about its nuclear program without preconditions," Hague said. "If not, we must continue to increase the pressure and we are considering with our partners a range of additional measures to that effect."
Hague said Iran's actions run counter to the gains made by the Arab Spring, if not worse.
"They may threaten to undermine it, bringing about a nuclear arms race in the Middle East or the risk of conflict," he said.
According to the report, Iran is believed to have continued weapons research and technology development after 2003, when the intelligence community thought it had stopped. Instead of halting, it seems Iran took a hiatus, although the program has progressed at a more modest pace since then, the report said.
Iran has always said its nuclear program is for peaceful purposes.
Iranian officials slammed the nuclear watchdog agency's report as a fabrication aimed at satisfying U.S. allegations about Iran's nuclear program.
"They are all armed to the teeth with nuclear weapons but, quite unashamedly, they accuse Iran of trying to acquire nuclear weapons," President Mahmoud Ahmadinejad said. "Know that this nation will not retreat an iota from its (position)."
A spokesman for the Foreign Ministry reiterated Iran's position that it is ready to come to the table but remains discouraged by what it perceives as outside pressure wrought on the IAEA.
"Negotiations will not work if there is outside pressure," Ramin Mehmanparast said. "If anyone thinks that they can create an advantage in negotiations by applying outside pressure, then they're making a mistake."
And Ali Asghar Soltanieh, the Iranian ambassador to the IAEA in Vienna, told CNN that Iran has been transparent. No other nation, he said, has allowed inspectors into facilities as Iran has. "We have nothing to hide."
"I assure you we do not have any nuclear weapons program," he said.
Soltanieh told CNN International's "Connect the World" that the report is not fair and balanced. And, he said, international sanctions have had no effect on his country's nuclear program.
"Let's stop this childish game," he said. "Let's come to the negotiating table."
After the latest report, however, top Republicans in the U.S. Congress called on President Barack Obama to ratchet up economic sanctions against Iran.
Rep. Mike Rogers, chairman of the House Intelligence Committee, raised the fear that Israel would attack Iranian nuclear facilities without further steps by the United States and its allies.
Israel, meanwhile, warned it will consider all options to confront its archnemesis.
"It's clear that Iran is trying to achieve a nuclear weapon and Iran needs to be stopped," said Tzipi Livni, an Israeli opposition leader and former foreign minister. "Iran needs to understand ... that all options are on the table."
Western powers have long suspected that Iran's nuclear program is geared toward weapons development. The United States is looking to increase the heat on Iran, including a possible strengthening of sanctions on Iran's financial and banking sectors and additional political pressure -- all of which could be applied by the United States alone or in coordination with other allies.
A statement from the French Foreign Ministry echoed British sentiment.
"France believes that it must make a breakthrough in strengthening diplomatic pressure on Iran," the statement said. "If Iran refuses to comply with the demands of the international community and denies any serious cooperation, we stand ready to adopt, with all the countries that will follow, sanctions of an unprecedented scale."
China Foreign Ministry spokesman Hong Lei said Iran bears the responsibilities of nuclear nonproliferation.
The United States also hopes international organizations, such as the United Nations, will take steps to further isolate Iran diplomatically.
However, Russian Deputy Foreign Minister Gennady Gatilov said Russia will not support new sanctions.
"The world community will see additional sanctions against Iran as an instrument of regime change in Tehran," Gatilov told the Interfax news agency Wednesday. "We cannot accept this approach."
CNN's Yousuf Basil, Reza Sayah, Haolan Hong and Kevin Flower contributed to this report.