US Secretary of State John Kerry blamed the Iranian government for a deadly attack on an opposition camp in Iraq, and voiced fears of another assault.
"We are deeply engaged in this. I am very concerned about the potential of another attack," Kerry told US lawmakers when asked about the February attack on a camp housing members of an exiled Iranian dissident group, the MEK.
The People's Mujahedeen of Iran (MEK) has accused Tehran of being behind the rocket and mortar attack on Camp Liberty, a former US military base near Baghdad housing about 3,000 members of the MEK, in which seven people died.
Kerry told the House foreign affairs committee he had raised the issue with Iraqi Prime Minister Nuri al-Maliki during a visit last month to Baghdad, and said he was "worried about the security situation" at the camp.
"It's the Iranian government that I believe was behind the attacks," he added.
The MEK was founded in the 1960s to oppose the shah of Iran, and after the 1979 Islamic revolution that ousted him it took up arms against Iran's clerical rulers. It says it has now laid down its arms and is working to overthrow the Islamic regime in Tehran by peaceful means.
Saddam allowed the MEK to establish a base called Camp Ashraf, northeast of Baghdad, after he launched the 1980-88 war with Iran.
But almost all MEK members in Iraq have now moved to Camp Liberty from Camp Ashraf as part of a UN-backed process to resettle them outside the country.
Kerry voiced frustration at the slowness of attempts to resettle the residents, saying "we have contacted countless countries; we have been refused by countless countries."
"We had worked out an arrangement with the Albanians to take about 250 people, but then the people in the camp themselves declined to go," he said. "So we're trapped in a kind of round robin."
But Shahin Gobadi, spokesman for the National Council of Resistance of Iran (NCRI), the so-called parliament-in-exile, denied camp residents had refused to move to Albania.
Two lists of names had been handed over to UN and US officials, he said in a statement, saying both the MEK and the NCRI "in writing have taken responsibility for all expenses of those who are to be resettled in in Albania."
Nurses had also been designated to travel with sick residents. "So far, not even one person has been resettled and no specific date has been set for their transfer to Albania," he added.