Vienna (CNN) -- Evidence of ongoing activities at an Iranian military base suspected of being involved in testing related to nuclear weapons makes inspectors eager to get there as soon as possible, the head of the International Atomic Energy Agency said Monday.
Speaking to reporters during the agency's board of governors meeting, IAEA Director General Yukiya Amano declined to provide details on whether the activities at the Parchin military base involve ongoing testing or efforts to remove evidence.
"But I can tell you that we are aware that there are some activities at Parchin and it makes us believe that going there sooner is better than later," Amano said.
IAEA inspectors had asked to visit the facility during a February trip to Iran but were rebuffed, according to the agency. Inspectors believe Iran may have used Parchin to test high explosives that could be used to detonate a nuclear weapon.
Inspectors visited the site twice in 2005, but did not go into the building that housed the test chamber, according to the IAEA.
Iran offered access to another site late in the February visit, Amano said. But the inspection team in Iran was not outfitted to examine Marivan, a site the IAEA believes may have been used to test elements of a nuclear weapon in 2003.
Iran has said its nuclear program is entirely peaceful, but Amano said Iran's failure to cooperate with international inspectors makes it impossible to be sure.
In fact, the agency "continues to have serious concerns regarding possible military dimensions" of Iran's nuclear program, Amano said.
Because Iran is not following an agreement to provide expanded information and broader inspection access to international inspectors, 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," he said in a statement preceding his news conference.
"The reasons of our concern is that we have information, credible information, overall credible information that indicates that Iran engaged in activities relevant to the development of nuclear explosive devices," he told reporters.
IAEA inspectors traveled to Iran in January and again in February to discuss the issue, Amano said, but failed to reach agreement and Iran refused to grant inspectors access to the military base that the agency believes may have been used to test explosives that could be used to detonate a nuclear bomb.
Iran has disputed that it refused inspectors access to the base, saying that officials had only demurred until Iran and the IAEA could reach agreement on details of the visit.
Monday's statement by Amano is not the first time the agency has questioned the purposes behind Iran's nuclear program, which the country has said is entirely peaceful.
Most recently, after the February visit by inspectors, the agency issued a report announcing that Iran had stepped up its efforts to produce enriched uranium in violation of international resolutions calling on it to stop. The agency expressed "serious concerns" about potential military uses by Iran in that report.
Among other things, Iran has tripled its monthly production of uranium enriched to contain a 20% concentration of radioactive material and taken other steps to ramp up its nuclear program, Amano said Monday.
While Iran has said the higher-level enrichment is meant to produce therapies for cancer patients and other peaceful purposes, international critics have called the efforts a troubling step toward possible militarization of nuclear technology there.
Nuclear weapons require concentrations of about 90%.
Amano said the agency would continue discussions with Iran and urged the country to abide by IAEA and United Nations resolutions on its nuclear program.
Iran is under increasing international pressure regarding its nuclear program.
The United Nations, the United States, the European Union and other countries have imposed sanctions related to Iran's nuclear research, and speculation regarding a possible military strike on Iranian nuclear facilities has been rampant in recent months.
U.S. President Barack Obama and Israeli Prime Minister Benjamin Netanyahu were scheduled to meet Monday at the White House to discuss Iran's nuclear program and the possibility of a strike by Israel.
On Sunday, Obama warned that "all elements of American power" remain an option to prevent Iran from developing nuclear weapons.
CNN's Tom Cohen contributed to this report.