The head of the U.N. nuclear agency expressed growing concern on Monday about investigating an Iranian site suspected of links to nuclear weapons development, saying there are indications of new activity there.

International Atomic Energy Agency chief Yukiya Amano did not specify whether he believed the activity was linked to suspected new weapons experiments or attempts to clean up previous alleged work. But he said the suspicions of "activities ... ongoing at the Parchin site" in Iran means "going there sooner is better than later" for IAEA inspectors seeking to probe suspicions that Iran has been -- or is -- working secretly to develop nuclear arms.

Inspecting Parchin was a key request made by senior IAEA teams that visited Tehran in January and February. Iran rebuffed those overtures, as well as attempts by the IAEA to question officials and secure other information linked to the allegations of secret weapons work.

Herman Nackaerts, a senior Amano deputy, told IAEA board members of such suspicions last week, referring to satellite images as his source, but the fact that Monday's comments came from the head of the agency added extra weight to the concerns.

Iran denies any intention of possessing nuclear weapons and says all of its atomic activities are peaceful, but the agency says it has intelligence-based suspicions that may not be the case based on thousands of pages of documentation.

Parchin is a key element. The agency says it may have been used to experiment with precision detonations normally used to set off a nuclear charge.

"We have our credible information that indicates that Iran engaged in activities relevant to the development of nuclear explosive devices," Amano told reporters outside of a 35-nation IAEA board meeting in Vienna, describing his sources as "old information and new information."

The conference opened as fears grow that Israel's air force may soon strike Iran in an attempt to destroy its nuclear facilities.

President Barack Obama met with Benjamin Netanyahu in Washington on Monday and told the Israeli prime minister that the United States "will always have Israel's back," but that diplomacy is the best way to resolve the crisis over potential Iranian nuclear weapons.

The U.S. and its Western allies went into Monday's IAEA meeting hoping to persuade Russia and China to back a resolution critical of Iran's refusal to heed IAEA and U.N. Security Council demands that it banish such concerns by opting for full nuclear transparency.

Moscow and Beijing traditionally act as brakes on Western attempts to tighten the sanctions vise on Iran, and a diplomat -- who asked for anonymity because his information was privileged -- told The Associated Press that the focus is on finding language they could agree with, without watering down the message to the point that it becomes meaningless.

However, another diplomat later said such attempts had been abandoned because the language rift was too great to bridge.

Any resolution passed by the IAEA board automatically goes to the U.N. Security Council and could be used as a platform for additional sanctions on the Islamic Republic, which already is the focus of four sets of U.N. sanctions meant primarily to pressure it to give up enrichment.

The U.S., the European Union and others have additionally slapped Tehran recently with financial and economic penalties meant to hurt its banking system and oil export industry.

Inside Monday's closed meeting, Amano summarized his worries: Tehran's rebuff of two recent attempts to probe the weapons program suspicions and a sharp, recent increase in uranium enrichment, which Iran says it needs for nuclear energy, but which can also produce fissile weapons material.

Recent moves to boost higher-enriched enrichment at Fordo, an underground facility that may be able to withstand aerial attack, are of particular concern.

Referring to his most recent report on Iran circulated late last month, Amano noted that Tehran had tripled higher monthly enrichment to 20 percent at Fordo over the past four months, as well as significantly expanding lower-level enrichment at another facility.

Both lower enriched uranium below 5 percent and 20 percent enriched material can be processed further to 90 percent -- the level used to arm nuclear warheads. But 20-percent enrichment is of particular concern because it can be turned into weapons-grade material much more quickly and easily that lower-enriched uranium.

"The agency continues to have serious concerns regarding possible military dimensions to Iran's nuclear program," he said, in comments made available to reporters. "As Iran is not providing the necessary cooperation ... the agency is unable to provide credible assurance about the absence of undeclared nuclear material and activities in Iran, and therefore to conclude that all nuclear material in Iran is in peaceful activities."

Outside the meeting, Ruediger Luedeking, Germany's chief IAEA delegate, told The Associated Press that onus was on Iran to "actively disprove the substantial doubt ... about the exclusively peaceful nature of its nuclear program."

The IAEA meeting comes less than two weeks after IAEA experts returned from Tehran from their second failed attempt within a month to persuade Iran to end nearly four years of stonewalling on what the agency says is growing intelligence-based information that Iran has worked -- and may still be working -- on components of a nuclear weapons program.

Iran dismisses the suspicions as based on fabricated information provided by the United States and Israel.