BAGHDAD, Iraq (AP) – Insurgents launched a brazen coordinated attack on an American combat post Monday, sending in a suicide bomber and clashing with American troops. Two American soldiers were killed and 17 wounded, the military said.

The assault began with a suicide bomber exploding a vehicle outside the base north of Baghdad, said the military statement. It gave no further details beyond the number of dead and wounded.

Residents said American forces fought with militants after the suicide bomber tried to break through barriers around the base near Tarmiyah, about 30 miles north of Baghdad. For hours, helicopters were seen landing at the base and leaving.

The attack came on a day when a string of car bombings and other attacks claimed more than 40 civilian lives in Baghdad and elsewhere.

Baghdad’s majority Shiite neighborhoods were the focus of the attacks Monday in the capital. In the worst strike, a mortar attack killed at least 11 people in the Dora area, a mostly Shiite enclave surrounded by predominantly Sunni neighborhoods.

Elsewhere in the capital, a suicide bomber donated an explosives-rigged belt on a public bus headed for the Karradah neighborhood, police reported. Five people were killed. Two other bombs – both in the southeastern Shiite neighborhood of Zafraniyah – killed a total of eight.

In Mahmoudiya, 20 miles south of the capital, a car bomb went off among auto repair shops, killing two and wounding two, police said. Mahmoudiya is mostly Shiite with Sunnis living in villages around the community and has long been a flashpoint for sectarian violence.

Outside the capital, a car bomb in Ramadi, about 90 miles west of Baghdad, killed at least nine bystanders congregated at a police checkpoint in the aftermath of a failed suicide attack. In Duluiyah, a Sunni area about 45 miles north of Baghdad, at least four were killed when a bomb-rigged car exploded.

The American military announced Monday that two American service members were killed in combat in western Iraq – an American Marine who died Saturday and a soldier who died Sunday in Anbar, a Sunni insurgent hotspot west of the capital.

The latest attacks were a sobering reminder of the huge challenges confronting any effort to rattle the well-armed and well-hidden insurgents.

A massive twin car bombing at a market in a mostly Shiite area on Sunday marked the first major blow to the American-led security crackdown in the capital, where Iraqi officials had boasted that militant factions were on the run. Police said at least 62 people died in that attack.

Just a few hours before, Lieutenant General Abboud Qanbar led reporters on a tour of the neighborhood near the marketplace and promised to “chase the terrorists out of Baghdad.” On Saturday, the Iraqi spokesman for the plan, Brigadier General Qassim Moussawi, said violence had plummeted 80 percent in the capital.

Prime Minister Nouri al-Maliki condemned the bombing as a desperate act by “terrorists” and “criminals” who sense they are being squeezed.

“These crimes confirm the defeat of these perpetrators and their failure in confronting our armed forces, which are determined to cleanse the dens of terrorism,” Mr. al-Maliki said in a statement.

American Ambassador Zalmay Khalilzad said the bombings underscore the “increasing desperation felt by criminals” and would only serve to “galvanize Iraqi forces and their coalition partners.”

The American-led teams have faced limited direct defiance as they set up checkpoints and comb neighborhoods. But that could change as they move into more volatile sections. The next could be Sadr City, a stronghold of the Mahdi Army militia loyal to radical Shiite cleric Muqtada al-Sadr.

American soldiers pressed closer to Sadr City and the reception changed noticeably. In previous days, Shiite families opened their doors to welcome the troops – feeling that the American presence would be a buffer against feared attacks from Sunni militia.

On Sunday, in areas closer to Sadr City, parents slapped away the candy and lollipops given by American soldiers.

“The Baghdad security plan is very important to push Iraq ahead,” said Haider al-Obeidi, a parliament member from the Dawa party of the prime minister al-Maliki.

Meanwhile, borders with Iran and Syria – shut for three days as the plan got under way – reopened Sunday. But new and strict rules will apply.

Mr. Moussawi was quoted in the Azzaman newspaper as saying the crossing points to the two nations would be open for only several hours a day and under “intense observation.”

America and allies claim Iraqi militants receive aid and supplies from Iran, including parts for lethal roadside bombs targeting American forces. Iran denies any role in trafficking weapons.