Donald Rumsfeld, the US Defence Secretary, accused Syria of arming President Saddam Hussein. He said the shipments, including nightvision goggles, were a direct threat to US and British forces and he added that Washington would hold Damascus accountable for “hostile acts” if the traffic continued.

Mr Rumsfeld said the movement of military supplies, equipment and people across the Syrian border “vastly complicates our situation”. Asked if he was threatening Damascus with military action, he replied: “I’m saying exactly what I’m saying. It was carefully phrased.”

Mr Rumsfeld also said that hundreds of revolutionaries of the Badr Corps, who are trained, equipped and directed by the Iranian Islamic Revolutionary Guard, were operating inside Iraq. He said American forces would be forced to treat them as enemy “combatants” and the Iranian Government would be held responsible for their actions.

The surprise threats raised the spectre that the war could suddenly and quickly spiral out of control. Arab opinion was further inflamed last night by reports that more than 50 civilians had been killed in an air raid that hit a market in the residential Baghdad neighbourhood of Shula.

Early today an enormous explosion rocked the centre of Kuwait City. The blast damaged a seafront shopping centre and cinema in the fashionable Souk Sharq district near the Kuwaiti parliament.

US military analysts said the missile was probably a Chinese-made Silkworm usually used to attack ships. Bystanders described the missile’s trajectory as low — Silkworm missiles are designed to fly low to avoid triggering early warning radar systems.

Yesterday Britain’s senior army commander warned Tony Blair that the British military was already overstretched; the huge commitment of troops to Iraq was “not sustainable over a long period”. General Sir Mike Jackson’s statement followed an admission by America’s ground commander in the Gulf that overextended supply lines and the enemy’s surprising resilience had meant the war could last longer than predicted.

“The enemy we’re fighting is different from the one we’d war-gamed against,” Lieutenant-General William Wallace said. The US is sending 120,000 troop reinforcements, which will double American combat power in Iraq.

Syria dismissed the US accusations. A statement from the Syrian Foreign Ministry said: “What Donald Rumsfeld said about the transportation of equipment from Syria to Iraq is an attempt to cover up what his forces have been committing against civilians in Iraq.”

Mr Rumsfeld’s warnings came after Syria hardened its opposition to the war this week. President Bashar Assad of Syria publicly expressed his hopes that Washington would fail in its mission to overthrow the Iraqi regime. In an interview published on Thursday in the Lebanese daily Al-Safir, Mr Assad predicted that if the US and Britain occupied Iraq, they would be met by “popular resistance” that would prevent them from controlling it.

He predicted that US troops would become bogged down in Iraq as they did in Vietnam, or forced to abandon the country as they did in the 1980s in Lebanon, which is now under Syrian dominance.

His words were published on the same day as a call by the country’s mufti for suicide attacks against US forces. The call by Sheikh Ahmad Kaftaro is unlikely to have come without the approval of Mr Assad’s regime.

Syria is the only Arab member of the UN Security Council. Although it voted for Resolution 1441, which led to the resumption of weapons inspections in Iraq, it has been vehemently opposed to war.

“America wants to remodel the region to its own liking,” Mr Assad said, echoing repeated criticisms from Damascus that the US is acting in the interests of Israel.

Mr Rumsfeld’s intervention was unprompted, coming at the end of his opening statement at the Pentagon’s daily press briefing. Quoting US intelligence, he said: “We have information that shipments of military supplies have been crossing the border from Syria into Iraq, including night-vision goggles. These deliveries pose a direct threat to the lives of coalition forces. We consider such trafficking as hostile acts and will hold the Syrian Government accountable.”

Moving on to Iran, Mr Rumsfeld said that any military forces, intelligence personnel or their proxies inside Iraq and not under the direct operational control of General Tommy Franks, the commander of US forces in the Gulf, would be taken as a potential threat to coalition forces.

Mr Rumsfeld’s deputy at the Pentagon, Paul Wolfowitz, became the first member of the Bush Administration to admit publicly that the US had failed to predict Saddam’s willingness to fight back.

Mr Wolfowitz, one of the key architects of the war, said: “We probably did underestimate the willingness of this regime to commit war crimes. I don’t think we anticipated so many people who would pretend to surrender and then shoot. I don’t think we anticipated such a level of execution squads inside Basra.”

A quarter of the British Army is in Iraq and General Jackson confirmed that there were contingency plans to send in reinforcements to replace exhausted troops if necessary. Half the Army is now committed to operations around the world and about 19,000 service personnel are tied up in the standby firefighting force in Britain.

The reports of more Iraqi civilian deaths yesterday, a day of prayer, came as coalition warplanes mounted a second consecutive day of intense bombing over Baghdad. Most of the explosions were in the south, apparently targeting Republican Guard units, but Shula is in the northwest of the city. Osama Sakhari, a doctor, said he counted 55 dead.

Pictures of the injured at a hospital were broadcast by al-Jazeera, the Qatar-based satellite channel, and al-Arabiya, based in Dubai. Both stations also showed pictures of Iraqi civilians vowing revenge.