World leaders worry that there could still be a massacre of civilians on a huge scale in Libya as Moammar Gadhafi's army continues to target rebel positions in several cities.

Seventeen people were killed Sunday as Gadhafi forces blasted the besieged city of Misrata with mortar rounds and rocket propelled grenades. Government troops have been firing into the rebel-held city for weeks, which has prompted calls for NATO forces to intensify air strikes on Gadhafi's forces.

In eastern Libya, government forces also shelled the eastern edge of Ajdabiya. So far, NATO air strikes have not stopped Gadhafi's relentless assault on rebel forces.

Meanwhile, the United Nations has reached an agreement with Gadhafi's government on providing humanitarian aid to the country, the U.N. Secretary General Ban Ki-moon said Monday.

Ban said the U.N. would set up a humanitarian presence in the Libyan capital of Tripoli. However, Libyan government spokesman Moussa Ibrahim said late Sunday the agreement also included setting up a humanitarian corridor to the city of Misrata, the last rebel stronghold in western Libya.

"The agreement is to provide safe passage for people to leave Misrata, to provide aid, food and medicine," Ibrahim said. He said the deal also called for free access of international aid agencies and ensuring that electricity, water and other services are provided to Misrata. City residents have said supplies have been disrupted by the fighting.

Ban said the U.N. already is providing aid in the rebel stronghold of Benghazi. He said the basic needs of tens of thousands of people in Libya are not being met.