BAGHDAD: Security officials in Baghdad were on Thursday urgently investigating the background of 30 Iranians who were caught fighting for a rebel Shia cleric in Iraq, amid mounting concern over the involvement of the Tehran regime in the uprising.

The London-based Guardian newspaper has learned that the most senior members of the Iraqi government were briefed about the capture of the men on Thursday, and also told of other evidence that fighters and equipment have been crossing the border from Iran.

The 30 men were captured in the southern city of Kut on Wednesday and officials are trying to establish whether they have any links to Tehran. "We are checking their identities but if they are found to have links to the Iranians then that would be tantamount to a declaration of war by them," said a senior Iraqi source.

The source said members of Iraq's national security committee had on Thursday been presented "with revealing information about the extent of Iranian involvement in Iraqi affairs", which was being taken seriously at the "highest echelons of government".

There was increasing frustration "at our neighbour's apparent indifference to cross-border security, despite promises of cooperation". The source said two trucks laden with weapons destined for the fighters of the militant cleric, Moqtada al-Sadr, had been stopped at the Iranian border on Wednesday night.

Sabbah Kaddim, a senior adviser at the ministry of the interior, declined to confirm the seizure of the two trucks or the arrest of the Iranians. But he confirmed "there were a number of non-Iraqi elements" captured in Kut".

He added: "There has been a continuous stream of vehicles over the last few weeks trying to ferry arms across the border from Iran. "We catch some, others must get through. The trouble is knowing who exactly is behind all this."

The violence between US and Iraqi forces and Mr Sadr's supporters has destabilized Shia areas of the capital and several cities across southern Iraq where Iranian influence is at its strongest.

Baghdad knows the unrest poses a critical test of strength for the interim administration of Ayed Allawi, whose success will be judged on the ability to deliver a secure environment in which to hold the country's first post-Saddam elections, scheduled for next January.

Iran denies stirring up violence in Iraq. It says it does not knowingly let fighters cross the long border between the two countries, but accepts that some might cross illegally. Foreign fighters account for only a fraction of the insurgents in detention in Iraq.

Relations between Iran and Iraq, who fought a ruinous war from 1980-88, have plummeted in recent weeks. Iran on Thursday summoned Iraq's top envoy in Tehran over the alleged arrest in Iraq of several reporters from Iran's state news agency and the fate of a kidnapped Iranian diplomat. Iran also denounced the assault by US marines and Iraqi forces in the holy Iraqi city of Najaf as "inhumane and horrible".

Foreign diplomatic observers in Baghdad have been alarmed by the "stoking up of tension" between the two neighbours. One senior diplomat said the Iranians were pursuing their activities in Iraq "more aggressively than three months ago, and they were hardly passive then".

Some foreign diplomats, however, question whether Iran would be able to do anything in Iraq than other than "stir things up a bit". An interior ministry official said on Thursday: "We do have problems, but we believe that we can take the problems to the Iranian side and discuss them." -Dawn/The Guardian News Service.