The US ambassador to Iraq said that Iran, despite professing support for the government of Iraqi Prime Minister Nuri al-Maliki, has deepened its support for Shia militias in the two months since the delegations last met.

"We have actually seen militia-related activity with Iranian support go up and not down," US Ambassador Ryan Crocker told reporters, referring to the period since the Iranian, Iraqi and US delegations last met on May 28.

"When there are results, the results that count will be results we see on the ground. Rather clearly we haven't seen those kind of results so far," he claimed, after several hours of talks with Iranian and Iraqi officials.

The talks began at 10:15 am (0615 GMT) at the office of Prime Minister Nuri al-Maliki, who began the meeting with a brief speech, according to an official at the Iraqi premier's office.

"We hope to see your support in stabilising Iraq. We do not want to see Iraq interfering in the affairs of others, nor do we want anyone to interfere in its internal affairs," Maliki said, according to a statement from his office.

Photographs from the meeting showed the three delegations sitting around a triangular arrangement of tables in conference room in Maliki's offices inside Baghdad's heavily-fortified Green Zone.

The US was represented by Crocker, while Tehran's envoy Hassan Kazemi Qomi headed the Iranian delegation in the talks attended by a delegation of Iraqi officials led by Foreign Minister Hoshyar Zebari.

As at May's meeting, officials said the talks had only dealt with the security situation in Iraq, leaving aside a roster of other disputes between the United States and the Islamic republic.

The United States broke off relations with Iran in 1980, when Islamic revolutionaries seized the US embassy in Tehran and held its diplomats hostage for 444 days.

The two countries remain at loggerheads over a range of issues including Iran's nuclear programme, which the United States claims is aimed at producing nuclear weapons, an accusation vehemently denied by Tehran.

US forces also accuse Iran of arming and training Iraqi militias, allegations Iranian officials have also denied, and which a government spokesman again rejected on Tuesday.

"These accusations are without basis and do not help the atmosphere of the negotiations," Iranian foreign ministry spokesman Mohammed Ali Hosseini told journalists in Tehran before the meeting.

"These declarations aim to deceive public opinion which is troubled by the United States' warlike policy," Hosseini said, adding that the "Americans would be better off finding ways to get out of the Iraqi crisis."

Relations have also been chilled by the detention in Iraq by US forces of at least five Iranian officials whom Tehran insists are diplomats, but Washington says are covert operatives from of Iran's elite Revolutionary Guard.