US President George W Bush has reiterated his belief that Iran has been involved in attacks on US troops in Iraq.

In his first news conference of 2007, Mr Bush said the US was 'certain' that Iran's Revolutionary Guards was providing explosive devices to militants.

He denied he was laying the groundwork for an attack on Iran.

Separately, US officials have claimed that the radical Shia cleric Moqtada al-Sadr has left Iraq for Iran ahead of a planned military crackdown on sectarian violence in Baghdad.

However this has been denied by aides to the Shia cleric who say he is currently in Najaf.

The cleric has not been seen publicly in Iraq in recent weeks, but Nassar al-Rubaie, head of Sadr's parliamentary bloc, insisted he is 'still inside Iraq and working normally'.

They claim that Mr Sadr supports the US-Iraqi security plan for Baghdad and that militias will be disbanded once order is restored in the capital.

Moqtada al-Sadr's Mahdi Army militia has been accused of having a role in the deaths of hundreds of Sunni civilian deaths.

Though hundreds of Mahdi Army fighters have been detained in recent weeks, many leaders are said to have left for Iran or Syria.

Borders closed

The Iraqi government said yesterday it would close its borders with Iran and Syria and lengthen a night curfew in Baghdad to try to curb unrelenting violence in the capital.

Of four posts on the Syrian border, one will close indefinitely, and three more be shut for at least 72 hours while new security measures are put in place.

All Iranian border crossings will be closed as well, with four reopening in three days after similar improvements.

Meanwhile, the US House of Representatives began a debate last night on the Bush administration's policy in Iraq.

It is considering a non-binding resolution opposing US President George W Bush's decision to send more than 20,000 extra troops there.