Washington - The US raid against a top al-Qaeda operative this week in Syria was the latest effort by the Bush administration to step up pressure on the terrorist network, even by striking in countries without the approval of their governments.

Syria strongly protested the helicopter-borne attack on Sunday, 8 kilometres beyond the Iraqi border, where the United States is believed to have killed an al-Qaeda lieutenant responsible for smuggling fighters, weapons and money into Iraq.

The White House has not publicly acknowledged the attack. Damascus says that only civilians were killed.

Syrian outrage after the strike mirrored similar complaints from Islamabad over a series of crossborder raids and strikes by US forces against al-Qaeda and Afghan Taliban militants who use the rugged, ungoverned region inside Pakistan near the border as refuge.

Washington appears to be ordering the attacks based on broad legal arguments that the US can act in self-defence to protect US and coalition soldiers in Iraq and Afghanistan, if neighbouring countries are unwilling or unable to crack down on militants, international law experts say.

"If the state is unable to control what is going on within its territory, then the case can be made that it relinquishes some of its sovereignty to those who can," Anthony Clark Arend, a professor of international law and diplomacy at Georgetown University in Washington, said in assessing the Bush administration's legal rationale.

Arend said he frequently disagrees with legal conclusions adopted by the Bush administration, but in the cases of the assaults in Syria and Pakistan, the White House's position is not "an unreasonable argument."

"But many legal scholars would say that this is per se illegal unless you can show that the state is supporting or encouraging" the militants.

Washington has long complained that Syria has not taken adequate steps to prevent the flow of militants and potential suicide bombers across its border into Iraq, but Sunday's attack, the boldest incursion so far by the US military into Syria, came as US generals had observed in the last year a sharp drop in infiltrations.

It also came as relations between the United States and Syria had started to mildly improve after Damascus agreed to indirect peace talks with Israel and announced plans to establish an embassy in Beirut, a key step toward recognizing Lebanese sovereignty.

John Pike, an analyst at Globalsecruity. org, said the Bush administration could be sending a signal to Syria that the positive steps Damascus has taken are not enough.

"Just because you're moving forward in one direction doesn't mean you're going to get a free ride," Pike said.

International law that allows nations to act in anticipation of self defence can be murky, sometimes outdated or often ignored, and there is a long history of countries waging military campaigns across borders.

The Turkish military regularly moves into northern Iraq to attack Kurdish rebels. Colombia launched a strike in March against FARC guerilla camps in Ecuador, sparking a diplomatic crisis between the two countries. In September 2007, Israeli warplanes destroyed a suspected nuclear reactor in Syria.

The Bush administration appears to be operating under Article 51 of the United Nations Charter, which allows countries to use military force in another country in self-defence.

The United States and many other nations have over time viewed the article as a legal basis for acting against non-state actors such as terrorists plotting attacks in another country, said Ruth Wedgwood, a professor of international law at Johns Hopkins University in Washington. The same UN article was the basis of the Security Council resolution authorizing military intervention in Afghanistan following the September 11, 2001, terrorist attacks, she said.

"You have to show that there was an imminent danger and that this was the method of intercepting the danger," Wedgwood said. "It's not something that should be lightly done."

US officials, speaking anonymously in media reports, have said the target of the raid was an Iraqi native who goes by the nom de guerre Abu Ghadiya. The US government in February designated him as a lead al-Qaeda facilitator of weapons and foreign fighters into Iraq.

Syrian officials said that eight civilians, including children, died in the raid on a farm and asked the United Nations to prevent further US strikes.

"We consider this an act of criminal and terrorist aggression," Syrian Foreign Minister Walid al-Muallem said.

The Bush administration has been frustrated by the inability of the Pakistani government, a close ally in the war on terrorism, to contain al-Qaeda and Taliban militants hiding in the Federally Administered Tribal Areas (FATA) in the Waziristan region.

The militants have used the refuge to launch crossborder attacks on US and NATO troops, as violence in Afghanistan intensified during the last year, adding to the sense of urgency to clamp down in the FATA.

The US military and CIA doubled flights by unmanned Predator drones armed with Hellfire missiles launched against suspected hideouts. A covert ground assault in September angered the Pakistani government and added tension to already cooling relations. The military has reportedly not used ground troops in Pakistan since.

Pike, the security analyst, said the Bush administration could have determined that it could not continue waiting for Syrian authorities to go after Ghadiya.

"Is the US government really going to sit back and let an insurgency get a free ride just because it happens to be a few thousand feet on the wrong side of a line on a map?" he said. "Not indefinitely." (dpa)