Ibrahim al-Jaafari, the Iraqi Prime Minister, and Robert Zoellick, the visiting US Deputy Secretary of State, blamed foreign fighters who infiltrate the country for the recent wave of violence, which has claimed the lives of some 500 people, many of them Iraqi civilians.

“We and others are watching how Syria is behaving itself,” Mr Zoellick said.

“We made it quite clear to Syria that there are a number of actions Syria needs to take . . . I hope Syria gets the message. Countries are responsible for what happens on their territory.” The remarks are part of a campaign to exert maximum pressure on the regime of President Assad, which bowed to international demands and removed its troops from Lebanon last month. The US now wants Syria to follow up by blocking the flow of men, arms and money into Iraq.

US and Iraqi officials claim to have detailed intelligence that Syria has ignored the activities of the insurgents, who run two main supply routes, one down the Euphrates river into the restive Anbar province and the other towards the northern city of Mosul.

A senior US military officer told journalists in Baghdad this week that intelligence gathered from recent arrests of insurgent suspects revealed that alZarqawi’s al-Qaeda group met in Syria a month ago and decided to step up attacks using car bombs to destabilise the country’s fledgeling Government. It was unclear whether the Jordanian attended the meeting, but the results were evident. There have been 21 car bombs in Baghdad in just this month compared with 25 last year.

“We are concerned that al-Zarqawi is supported by a foreign fighter network that gets foreign fighters largely through Syria,” an official told journalists in Washington. “There are locations in Syria where foreign fighters and money and logistics come together and then transit to Iraq and those foreign fighters and many come from elsewhere in the Muslim world.”

Syria dismissed the allegations yesterday as “baseless”. An official in Damascus insisted that the country was doing all it could to stop cross-border activity on the long, rugged frontier.

But a senior Iraqi official said that the Syrians had been confronted with “details” about insurgent activities on their soil at a recent meeting in Istanbul. “They said that they would take action, but they’re absolutely not co-operating,” he told The Times. “I warned them that they were playing Russian roulette.” Last week US Marines began an offensive along the Iraqi-Syrian border and faced heavy resistance near the town of al-Qaim, where 14 Americans and 125 insurgents were killed. At one point the fighting came to within two miles of the Syrian border, where many of the insurgents fled.

There are fears that unless the situation is addressed quickly, the violence could spill over the border. The wave of attacks, which continued yesterday with the killings of an Oil Ministry official and seven members of a politician’s family in Mosul, prompted the senior US military officer to issue a warning that the struggle could continue for “many years”.The officer said: “It is much more likely to succeed, but it could still fail.”

He added: “If we fail, the different groups would be at each other’s throats and warfare would continue for some time. If we take our foot off their throats, this country could be back into civil war and chaos.”

Times Recommends

• The Japanese Prime Minister is doomed: or is he?
• Brighton company 'involved in cyber attack'
• The art of survival: it’s all in the mind