Baghdad, Iraq ? Three U.S. service members were killed and scores were wounded Sunday in rocket attacks on the fortified Green Zone and a military base in Baghdad, the U.S. military said.

A fourth U.S. soldier was killed by a roadside bomb in Diyala province, the military said.

The rocket attacks came at 3:30 p.m., according to a U.S. military official who spoke on condition of anonymity because he was not authorized to release the information. The attack on the Green Zone killed two of the soldiers and wounded 17, the official said. The other attack in the city, at a U.S. military base in Rustamiyah in eastern Baghdad, killed one soldier and wounded 14, the official said.

“It’s a tough day for us,” the official said. “These are our colleagues, our friends.”

The Green Zone and U.S. military facilities have become frequent targets of rockets and mortar shells that military officials say are fired from Sadr City and other parts of eastern Baghdad.

The latest series of attacks began late last month in response to an Iraqi government military offensive against Shiite militias in Basra, in southern Iraq. A cease-fire negotiated between deputies of radical Shiite cleric Muqtada al-Sadr, who wields extraordinary influence among many Shiites, and the Iraqi government restored order last week.

But the entrances to Sadr City have been tightly guarded, with few vehicles allowed to enter or leave. Residents of the vast slum have been on edge in recent days, as Prime Minister Nouri al-Maliki has given mixed signals about whether raids in the area are imminent. Sadr, a former political supporter of al-Maliki, had a falling-out with the prime minister last year and has stepped up his criticism of what he calls the U.S. occupation of Iraq.

It was unclear whether the attacks were related to clashes in Sadr City early Sunday. Nine men were killed when U.S. military helicopters fired Hellfire missiles into Sadr City, after a week of relative calm in an area that has become a flash point of violence in Iraq, the U.S. military said.

U.S. soldiers fired two missiles at 8 a.m. after an Iraqi army team came under rocket-propelled grenade fire, said Lt. Col. Steve Stover, a U.S. military spokesman. Three men were killed by the first missile. The others died after the second missile hit their getaway vehicle.

An Iraqi army spokesman said 11 people were killed in the fighting.

Also Sunday, 42 university students traveling in a bus from Baghdad to Mosul were kidnapped at gunpoint, U.S. and Iraqi officials said. U.S. soldiers in a helicopter spotted the bus near Mosul and helped Iraqi soldiers stop it shortly after the abductions.

Two suspected insurgents were detained, the U.S. military said. No students were hurt.