A weeklong offensive targeting suspected insurgents in Iraq near the Syrian border wrapped up Sunday with military officials calling it a success despite heavy resistance.

Operation Matador, which saw some 1,000 Marines and soldiers sweep through towns along the Euphrates River, resulted in more than 125 suspected insurgents killed and 39 captured, the military said. But it also cost the lives of nine Marines, with 40 more wounded.

Six of the Marines were killed when their amphibious assault vehicle hit a mine.

The large-scale operation was aimed at smuggling routes and safe houses for foreign fighters arriving in Iraq through the western desert border area, U.S. military officials said. The cities of Karabilah, Ramana and Ubaydi have been “used as a staging area where terrorists receive weapons and equipment and organize for attacks against the key cities of Ramadi, Falluja[h], Baghdad and Mosul,” read a Multi-National Force-Iraq statement issued Sunday.

“Regimental Combat Team-2 started and ended this operation as planned, accomplished its mission and secured all objectives. Coalition and Iraqi Security Forces will return again to this area in the future,” Maj. Gen. R.A. Huck, 2nd Marine Division commander, was quoted as saying in the release.

The offensive included armored ground troops, attack helicopters, and Harrier and Hornet fighter bombers. Marines reported encountering some insurgents fighting in organized units and with advanced equipment including body armor.

The operation kicked of May 7, when Marine-led U.S. forces crossed the Euphrates River to clear a series of caves suspected of being used as supply points or safe havens.

More than 70 suspected insurgents were killed in the first 24 hours of the battle, according to the U.S. military. On Saturday, a long convoy of U.S. military vehicles left the area and headed back toward their bases.

But, according to the Associated Press, the insurgents returned just as quickly. In Qaim, near where the operation kicked off, masked insurgents armed with machine guns and rocket-propelled grenades “remained in plain sight,” set up checkpoints and vowed to “defend the town if U.S. forces return,” the AP reported.

In an article published in Sunday’s Washington Post, an embedded reporter from the newspaper quoted Marine ground commanders as claiming success but being somewhat frustrated by the enemy after the first few days of the campaign.

“That was the frustrating piece: coming up here for a fight and not finding anyone,” Marine Maj. Steve Lawson, of the 3rd Battalion, 25th Marine Regiment, was quoted as saying.

Another commander, Lt. Col. Tim Mundy, said many of the fighters fled the area instead of standing to fight.

“The mission was to put on the pressure and show they did not have a safe haven from us. They ran from us,” he said.

While the offensive in western Iraq was under way, a wave of suicide attacks struck central and northern Iraq, killing more than 400 people, including security forces and civilians, in one of the worst outbreaks of violence since the 2003 invasion.

U.S. military and political leaders attribute the surge in attacks to attempts to destabilize the recently seated Iraqi cabinet.

Information obtained by The Associated Press and The Washington Post was used in this report.