VIENNA - Iran has tripled its monthly production of higher-grade enriched uranium and the UN nuclear watchdog has "serious concerns" about possible military dimensions to Tehran's atomic activities, the agency's chief said on Monday.Yukiya Amano, director general of the International Atomic Energy Agency, also told the IAEA's 35-nation board of governors about the lack of progress in two rounds of talks between the Vienna-based UN agency and Tehran this year.During meetings in the Iranian capital in January and February, Iranian officials stonewalled the IAEA's requests for access to a military site seen as central to its investigation into the nature of the Islamic state's nuclear activity."The agency continues to have serious concerns regarding possible military dimensions to Iran's nuclear programme," Amano told the closed-door meeting, according to a copy of his speech.The IAEA "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 added.A report by the IAEA to member states last month said Iran was significantly stepping up uranium enrichment, a finding that sent oil prices higher on fears that tensions between Tehran and the West could boil over into military conflict.Since the IAEA's previous report in November, Amano said Iran has tripled monthly production of uranium refined to a fissile concentration of 20 percent - well above the level usually needed to run nuclear power plants.Though indicated by the IAEA's confidential report last month, it was the first time Amano spoke in public about this rapid increase in Iran's enrichment activities, which has stoked Western and Israeli suspicions about Tehran's nuclear agenda.Despite intensive discussions with Iran, Amano said, there had been no agreement on a "structured approach" to resolve outstanding issues with its nuclear programme during the talks held in January and February.Iran "did not address the agency's concerns in a substantive manner," Amano said.Making clear, however, that he would keep trying to engage Iran on the issue, he added: "Regarding future steps, the agency will continue to address the Iran nuclear issue through dialogue and in a constructive spirit."Amano: Syria asked IAEA to understand 'delicate situation'Amano also said that Syria had asked the UN nuclear watchdog for understanding of the country's "delicate situation" in response to requests for Syrian cooperation with an investigation into suspected illicit nuclear activity.The Syrian comments cited by Amano were an apparent reference to Syrian President Bashar Assad's campaign to stamp out a popular uprising, in which over 7,500 people have died by a UN count. Amano made clear that no progress had been made in the UN agency's almost four-year-old investigation regarding Syria. The IAEA has been seeking access to a desert site at Deir al-Zor that US intelligence reports say was a nascent, North Korean-designed reactor intended to produce plutonium for atomic weaponry before Israel bombed it to rubble in 2007.The Vienna-based watchdog has also been seeking information about other sites that may have been linked to Deir al-Zor.Amano said he had written a letter to Syria in November last year urging it to address the agency's questions."I received a reply from Syria dated 20 February 2012, which asked for understanding of 'the difficult circumstances and the delicate situation that Syria is passing through,'" Amano said, according to a copy of his speech to the closed-door meeting."The letter pledged that Syria would continue to cooperate with the Agency to resolve outstanding issues."Syria says Deir al-Zor was a non-nuclear military facility but the IAEA concluded in May 2011 that it was "very likely" to have been a reactor that should have been declared to inspectors.In June last year, IAEA governors voted to report Syria to the UN Security Council, rebuking it for failing to cooperate with the agency's efforts to get concrete information on Deir al-Zor and other sites. Russia and China opposed the referral, highlighting divisions among the major powers."The agency continues to seek full access to other locations which the agency believes are functionally related to the (Deir al-Zor) site," Amano said. "I urge Syria to cooperate fully with the agency in connection with unresolved issues related to the Deir al-Zor site and other locations."US President Barack Obama and Prime Minister Binyamin Netanyahu were to meet shortly in Washington to discuss Iran, deeply at odds over the timing for possible last-resort military action against Iran's nuclear program.Even though Obama offered assurances of stiffened US resolve against Iran before the White House meeting, the two allies remained far apart over explicit nuclear "red lines" that Tehran should not be allowed to cross.