The raid, which a U.S. military official in Washington confirmed, indicated the desert frontier between the two countries remains a key battleground 5 1/2 years into the Iraq war. The U.S. official said the attack targeted elements of a robust foreign fighter logistics network and that due to Syrian inaction the U.S. was now "taking matters into our own hands."

A government statement carried by the official Syrian Arab News Agency condemned the raid and said the Foreign Ministry had summoned the charges d'affaires of the United States and Iraq to protest the strike.

The attack occurred just before sundown in an area of farms and brick factories about eight kilometers, or five miles, inside the Syrian border.

Four helicopters flying along the Euphrates River struck the Sukkariyeh Farm near the town of Abu Kamal, the Syrian government said. The helicopters attacked a civilian building under construction, firing on the workers inside and killing a number of civilians, including four children, the statement said.

The U.S. military official said the special forces raid targeted elements of a network that sends fighters from North Africa and elsewhere in the Middle East to Syria, where elements of the Syrian military are in league with Al Qaeda and other fighters. He spoke on condition of anonymity because of the political sensitivity of cross-border raids.

While U.S. forces have had considerable success in shutting down the "rat lines" in Iraq with help from Iraq and governments in North Africa, the Syrian part of the network has been out of reach, he said.

"The one piece of the puzzle we have not been showing success on is the nexus in Syria," the official said. "We are taking matters into our own hands."

The White House in August approved similar special forces raids across the border of Pakistan to target Al Qaeda and Taliban operatives. At least one has been carried out.

A resident of the nearby village of Hwijeh, speaking on condition of anonymity because of the sensitivity of the information, said some of the aircraft in Sunday's attack landed and disgorged troops who fired on a building. At least one of the dead was a construction worker, he said.

"Syria condemns this aggression and holds the American forces responsible for this aggression and all its consequences," the government statement said. "Syria also calls on the Iraqi government to shoulder its responsibilities and begin an immediate investigation into this serious violation and prevent the use of Iraqi territory for aggression against Syria."

The flow of foreign fighters into Iraq has been cut to an estimated 20 a month, a senior U.S. military intelligence official said in July. That marked a 50 percent decline from six months earlier, according to the official, who spoke on condition of anonymity in order to discuss intelligence reports.

Ninety percent of the foreign fighters enter through Syria, and foreign fighters toting cash have been Al Qaeda in Iraq's chief source of income, according to U.S. intelligence.

On Thursday, the commander of U.S. forces in western Iraq said in a briefing with Pentagon reporters that American troops were redoubling efforts to secure the Syrian border.

Major General John Kelly said in last week's briefing that Iraq's western borders with Saudi Arabia and Jordan were fairly tight as a result of good policing by security forces in those countries but that Syria was a "different story."

"The Syrian side is, I guess, uncontrolled by their side," Kelly said. "We still have a certain level of foreign fighter movement."

He added that the U.S. was helping construct a sand berm and ditches along the border.

"There hasn't been much, in the way of a physical barrier, along that border for years," Kelly said.

Foreign Minister Walid al-Moallem of Syria accused the United States earlier this year of not giving his country the equipment needed to prevent foreign fighters from crossing into Iraq. He said Washington feared Syria could use such equipment against Israel.

Syria's interior minister, Bassam Abdul-Majid, told a security meeting in the Syrian capital in April that it had stepped up measures along its border with Iraq, setting up fixed checkpoints and patrols to prevent insurgent infiltration and smuggling.

Though Syria has long been viewed by the U.S. as a destabilizing country in the Middle East, in recent months, Damascus has been trying to change its image and end years of global seclusion.

Its president, Bashar Assad, has pursued indirect peace talks with Israel and has agreed to establish diplomatic ties with Lebanon, a country it used to dominate both politically and militarily.