TRIPOLI, Libya – Forces loyal to Moammar Gadhafi 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 Gadhafi to meet demands for change from the opposition. However Turkish Prime Minister Recep Tayyip Erdogan adopted a much tougher stance Tuesday, saying that Gadhafi 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 Gadhafi’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 Gadhafi’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 Gadhafi’s efforts to retake rebel territory, but the ill-trained and badly equipped opposition has been unable to press the advantage and make advances.

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 Gadhafi’s forces for two months, fighting continued on the city’s edges.