"We are taking matters into our own hands," the official told The Associated Press in Washington, speaking on condition of anonymity because of the political sensitivity of cross-border raids.
The attack came just days after the commander of U.S. forces in western Iraq said American troops were redoubling efforts to secure the Syrian border, which he called an "uncontrolled" gateway for fighters entering Iraq.
A Syrian government statement said the helicopters attacked the Sukkariyeh Farm near the town of Abu Kamal, five miles inside the Syrian border. Four helicopters attacked a civilian building under construction shortly before sundown and fired on workers inside, the statement said.
The government said civilians were among the dead, including four children.
A resident of the nearby village of Hwijeh said some of the helicopters landed and troops exited the aircraft and fired on a building. He said the aircraft flew along the Euphrates River into the area of farms and several brick factories. The witness spoke on condition of anonymity because of the sensitivity of the information, Syria's Foreign Ministry said it summoned the charges d'affaires of the United States and Iraq to protest against the strike.
"Syria condemns this aggression and holds the American forces responsible for this aggression and all its repercussions. Syria also calls on the Iraqi government to shoulder its responsibilities and launch and immediate investigation into this serious violation and prevent the use of Iraqi territory for aggression against Syria," the government statement said.
The area targeted is near the Iraqi border city of Qaim, which had been a major crossing point for fighters, weapons and money coming into Iraq to fuel the Sunni insurgency.
Iraqi travelers making their way home across the border reported hearing many explosions, said Farhan al-Mahalawi, mayor of Qaim.
On Thursday, U.S. Maj. Gen. John Kelly said 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.
The foreign fighters network sends militants from North Africa and elsewhere in the Middle East to Syria, where elements of the Syrian military are in league with al-Qaida and loyalists of Saddam Hussein's Baath party, the U.S. military official said.
He said that while American forces have had considerable success, with Iraqi help, in shutting down the "rat lines" in Iraq, and with foreign government help in North Africa, the Syrian node has been out of reach.
"The one piece of the puzzle we have not been showing success on is the nexus in Syria," the official said.
The White House in August approved similar special forces raids from Afghanistan across the border of Pakistan to target al-Qaida and Taliban operatives. At least one has been carried out.
The flow of foreign fighters into Iraq has been cut to an estimated 20 a month, a senior U.S. military intelligence official told the Associated Press in July. That's a 50 percent decline from six months ago, and just a fifth of the estimated 100 foreign fighters who were infiltrating Iraq a year ago, according to the official.
Ninety percent of the foreign fighters enter through Syria, according to U.S. intelligence. Foreigners are some of the most deadly fighters in Iraq, trained in bomb-making and with small-arms expertise and more likely to be willing suicide bombers than Iraqis.
Foreign fighters toting cash have been al-Qaida in Iraq's chief source of income. They contributed more than 70 percent of operating budgets in one sector in Iraq, according to documents captured in September 2007 on the Syrian border. Most of the fighters were conveyed through professional smuggling networks, according to the report.
Iraqi insurgents seized Qaim in April 2005, forcing U.S. Marines to recapture the town the following month in heavy fighting. The area became secure only after Sunni tribes in Anbar turned against al-Qaida in late 2006 and joined forces with the Americans.
Syrian Foreign Minister Walid al-Moallem 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.
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, mediated by Turkey, and says he wants direct talks next year. Syria also has agreed to establish diplomatic ties with Lebanon, a country it used to dominate both politically and militarily, and has worked harder at stemming the flow of militants into Iraq.
The U.S. military in Baghdad did not immediately respond to a request for comment after Sunday's raid.