MISRATA, Libya – Moammar Gadhafi’s forces showered the port area of this besieged rebel city with rockets just minutes after an international aid ship docked there Wednesday, killing four people as part of the regime’s increasingly bloody attempt to choke off Misrata’s remaining lifeline.

The attack caused panic among hundreds of stranded migrant workers and fleeing Libyans who were trying to board the Red Star One, a ferry chartered by the International Organization for Migration. In the chaos, some families were separated and the boat had to redock twice to sort it all out.

The timing of the shelling suggested Libyan forces were deliberately trying to disrupt the evacuation. Othman Belbeisi, an IOM official, said rockets started flying just minutes after he and others first stepped off the ferry. “The whole place was shaking and people started running in different directions,” he said.

A State Department spokesman said the regime should cease hostilities in Misrata and allow the IOM to provide relief to civilians. He said the U.S. was making available an additional $6.5 million in assistance to IOM to help evacuate people from Misrata and for other operations in Libya.

On Tuesday, a senior Libyan official said the Libyan army would do what is necessary to block sea access to Misrata, already besieged by land for the past two months. Last week, Gadhafi’s troops were caught laying sea mines, disrupting shipping as NATO struggled to clear a safe access route to Misrata’s port. The Red Star One had waited at sea for three days before risking the approach on Wednesday, aided by a tug boat.

Misrata is the main rebel stronghold in western Libya, which remains largely under Gadhafi’s control, while the rebels have been holding on to most of the east. Since the uprising against Gadhafi broke out in mid-February, the two sides have largely been locked in a stalemate. The international community’s bombing campaign, launched in mid-March, has kept Gadhafi’s forces from advancing to the east, but has failed to give the rebels a clear battlefield advantage.

In Benghazi, the opposition stronghold in eastern Libya, rebel military spokesman Col. Ahmed Bani depicted the bombing of Misrata’s port as a crime against humanity.

In Europe, support for giving funds to the Libyan rebels — presumably to buy arms, equipment and munitions with which to overthrow Gadhafi — seemed to be growing as distaste for a long air war increased. Officials from countries involved in the military campaign will likely announce ways to help the rebels financially as they meet Thursday in Rome.

At the United Nations, meanwhile, the prosecutor of the International Criminal Court said he has evidence that Gadhafi’s security forces have systematically attacked civilians in trying to crush the armed uprising.

Luis Moreno-Ocampo told the U.N. Security Council 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.