By Jim Muir
BBC News, Baghdad
The US ambassador in Baghdad has cautioned of the dangers of over-hasty withdrawals of military forces in Iraq.
The violence has fallen, but anti-US forces remain
Ryan Crocker's warning comes as Democratic candidates in the US presidential race talk of setting timetables for pulling out troops.
The ambassador told the BBC a recent improvement in the current security situation was not irreversible.
He warned a new cycle of violence could start up if withdrawals were not handled very carefully.
Democratic contender Barack Obama has said he favours a full withdrawal by the end of next year, while Hillary Clinton says she will set a timetable within two months if she is elected.
Ambassador Crocker did not comment directly on the candidates and their views, but he stressed his own long-standing conviction that future withdrawals should depend on conditions on the ground - not on timetables set in Washington.
To remove that consideration, he said, would be to take an unacceptable risk.
"For all the progress that has been achieved over the past year there are still a strong sectarian tensions," Mr Crocker said.
Mr Crocker became the ambassador in Baghdad in March 2007
"There is still an al-Qaeda presence, there is still foreign interference particularly from Iran.
"Now all of these things can spark a new cycle of instability and violence if we're not very very careful and closely linked with the Iraqis on what the next step should be."
The current American troop surge is to be finished by July. After that, US military commanders and Mr Crocker advocate a pause for assessment before reducing troop numbers below the pre-surge levels of around 130,000.
The ambassador said Iran was continuing to support extremist militias in Iraq despite official support for the Iraqi government.
He hoped the visit by the Iranian President, Mahmoud Ahmadinejad, in early March would prompt Iran to bring its practices into line with its policies.