TRIPOLI, Libya -- Forces loyal to Muammar al-Qaddafi shelled a rebel supply route and a besieged opposition stronghold in western Libya on Tuesday, even as the embattled Libyan leader's international isolation deepened with a demand by Turkey that he resign now.

Turkey is a key regional mediator and in the past tried to nudge Qaddafi to meet demands for change from the opposition. However Turkish Prime Minister Recep Tayyip Erdogan adopted a much tougher stance Tuesday, saying that Qaddafi must "immediately step down."

In Tripoli, Libya's deputy foreign minister, Khaled Kaim, said such a decision is not up to Turkey, but the Libyan people. "If you want to be a mediator, you shouldn't express yourself to support one party against the other," he said.

NATO said its warplanes would keep up the pressure on Qaddafi's regime for as long as it takes to end the violence in the North African nation. Italian Navy Vice Adm. Rinaldo Veri said that NATO, having disrupted the regime's ground forces on the front lines, was now focusing on cutting Qaddafi's lines of communications with his troops.

Early Wednesday, two loud booms were heard in Tripoli, several minutes apart, apparently from NATO air strikes.

The bombing by a U.S.-led international force started seven weeks ago. NATO took over command of aerial operations at the end of March. The bombing campaign has stymied Qaddafi's efforts to retake rebel territory, but the ill-trained and badly equipped opposition has been unable to press the advantage and make advances against government forces.

Rebels in western Libya, reached by telephone, said loyalist forces fired dozens of rockets at a road Tuesday to disrupt supplies transported from Tunisia through the rebel-controlled Dhuheiba border crossing to rebel towns in a nearby mountain area of Libya. Shelling has caused the road to close intermittently.

The Libyan leader has been fighting rebels in the east of his vast oil-rich nation since an uprising against his rule began in February. His forces control most of western Libya save for a string of villages along the mountainous western border and the port city of Misrata.

In Misrata, under siege by Qaddafi's forces for two months, fighting continued on the city's edges, where regime loyalists have taken up positions. The two sides fired shells, and medics reported one dead and 22 wounded, most of them rebel fighters.

Last week, NATO vessels spotted Libyan forces trying to lay sea mines in the approaches to Misrata's harbor, in an attempt to choke off the city's only lifeline. Two of the mines were destroyed, but a third broke free. The hunt for the mines has disrupted shipping near Misrata in recent days.

The International Organization for Migration said it was desperately trying to get permission from NATO for its ship, the Red Star One, to dock at Misrata so that it can evacuate some 1,000 migrants and wounded civilians from the city.

Misrata, a city of some 300,000 people, is entirely dependent on food and medical supplies arriving from the sea. The city has also been intensively shelled by loyalist troops.

IOM spokesman Jumbe Omari Jumbe said NATO was holding back permission for the ship to dock even as 36 people need urgent evacuation. Nine of the 36 were in Misrata hospital's seven-bed intensive care unit, and two seriously ill civilians have died while waiting for the ship to pick them up, he said.

Asked about the mine-laying, Kaim, the Libyan government official said Tuesday that he did not know details. However, he said, "there was an announcement from the Libyan government, the ministry of transportation, that Misrata port is closed, and any foreign ship or vessel would be targeted by the Libyan armed forces."

Earlier this week, government officials had been evasive when asked whether Libyan authorities were effectively trying to block access to Misrata by sea.

Also Tuesday, the general in charge of Libya's rebel forces said a car bomb exploded in front of their headquarters in northeast Benghazi. No injuries were reported by Gen. Abdel-Fattah Younis.

The U.N. refugee agency, meanwhile, said Tuesday that fighting in the Libya-Tunisia border region has caused an uptick in the number of civilians fleeing Libya into southern Tunisia after a brief interruption last week.

UNHCR spokesman Adrian Edwards said some 8,000 people, mostly ethnic Berber women and children, arrived in Dhuheiba over the weekend. Migrants, many from sub-Saharan Africa, are also crossing the Mediterranean to Italy again, he said.

About 3,200 people arrived on the Italian island of Lampedusa during the past five days.

Outside pressure for Qaddafi to resign significantly increased on Tuesday. Erdogan told a news conference in Istanbul that Qaddafi has ignored calls for change in Libya and instead preferred "blood, tears and pressure against his own people."

"Qaddafi must take a historic step and withdraw, for the future of Libya, its peace and prosperity," he said.

Turkey initially balked at the idea of military action in Libya, but citing its responsibilities as a NATO member it took part in the enforcement of an arms embargo on Libya while volunteering to lead humanitarian aid efforts.

Turkey has vast trade interests in Libya, where Turkish companies have been involved in lucrative construction projects worth billions of dollars, building hospitals, shopping malls and five-star hotels before the uprising and resulting chaos began.

The Swiss government, for its part, said Monday it had identified 360 million Swiss francs ($415 million) of potential assets to be frozen belonging to Qaddafi or his entourage.

Kaim said Qaddafi does not have personal funds abroad, and that money in foreign accounts is part of Libya's investment portfolio. "We are ready to give it away if there is any money that belongs to the leader," he said.