Moammar Gadhafi's forces shelled the port area of this besieged rebel city on Wednesday, killing a woman, a man and two children as an international aid ship was docked, a medical official said.

The ferry, chartered to evacuate nearly 1,000 migrant workers, had spent three nights outside Misrata waiting to come in. Last week, Gadhafi's forces were caught mining the harbor entrance, and the Red Star One was guided into the port by tug boat Wednesday to avoid mines.

The four people killed in the attack had been in a nearby camp for stranded migrant workers, most of them African, said Mahdi Bensasi of the Libyan Red Crescent.

Saka Yossie, 27, a worker from Ghana, said he was in the camp when Grad rockets hit. "We were just sitting there when the bombs came down," he said. "They died right in front of us."

Yossie said he'd spent six weeks in the camp, sleeping in a tent with very little food and water. "Now I thank God for taking me from this place," he said stepping aboard the ferry, chartered by the International Organization for Migration

Two other rocket attacks struck Misrata's port earlier this week. The city of 300,000 is the biggest rebel stronghold in western Libya, an area largely controlled by Gadhafi. The rebels have been holding most of the east of the country.

Regime loyalists also stepped up attacks on rebels in a remote western mountain area, near Libya's border with Tunisia, shelling two towns and a key supply route Wednesday, opposition fighters said.

At the United Nations, meanwhile, the International Criminal Court prosecutor said evidence shows that Gadhafi's security forces have systematically attacked civilians in trying to crush an armed uprising that erupted in February.

The prosecutor, Luis Moreno-Ocampo, said he would seek arrest warrants in coming weeks against three Libyans who he said appear to bear "the greatest criminal responsibility" for crimes against humanity. He did not name them.

He told the U.N. Security Council that Gadhafi's forces have been systematically shooting at peaceful protesters, using the same tactics in multiple locations. He cited reports that civilians in Tripoli, the capital, and other government-controlled areas are subject to systematic arrests, torture, killings and enforced disappearances.

Misrata, under siege by land for more than two months, has been especially hard-hit.

Late Tuesday, Libya's deputy foreign minister, Khaled Kaim, confirmed that the regime is trying to cut off Misrata's access to the sea — its final lifeline.

Citing an announcement from the Ministry of Transportation, he said Misrata's port would be closed and "any foreign ship or vessel would be targeted by the Libyan armed forces."

Last Friday, two of the anti-ship mines placed the regime along the route to the port were destroyed, but a third floated away, and NATO vessels have been searching for it. The attempted mining disrupted the delivery of desperately needed supplies to Misrata.

The port area came under rocket fire Wednesday as soon as the IOM vessel had docked.

Shortly after the attack, a few migrant families arrived at the ship in the back of a pickup truck with their belongings. Then dump trucks arrived, their backs packed with hundreds of male migrants, mostly from Niger and other impoverished African countries. The migrants poured out of the trucks and raced to board the ship, many of them carrying no luggage.

The ship's chaotic departure highlighted the desperation of some in this once-wealthy city, Libya's third largest, after two months of war and daily shelling by forces loyal to Gadhafi.

Thousands of migrant workers were stranded in Misrata when the anti-Gadhafi uprising that has now split the country broke out Feb. 17. Most of the about 1,000 who remained had been living in a squalid tent camp near the port, waiting for a boat to take them out.

While the migrants filed on board Wednesday, some 300 Libyans, mostly families with children, crowded around the ship's back, hoping to flee the city.

Mohammed Iksibat, 48, came with his wife and two children, a 5-year-old girl and a 2-year-old boy.

"They keep hitting the city with heavy weapons and there is no place to be safe," he said, explaining why he sought to flee the city he's always lived in.

Trip organizers tried to keep the Libyan families from boarding, but about 100 rushed the boat, hauling their luggage aboard and ignoring calls to stop.

A rebel helping organize the evacuation fired one shot in the air to disperse the crowd and the boat suddenly pulled out to sea, separating some families and leaving medics and journalists behind.

Before the ship left, Mahdi Bensasi, the Red Crescent official, said 931 migrants remained to be evacuated from Misrata.

"They have to leave," Bensasi said. "We have no safe place in Misrata to keep them."

Those traveling were not counted or registered before they boarded as they were on previous trips.