Misrata is the only city in Libya's west that's still partly in rebel hands.
The fighting in Libya began in mid-February when large anti-government protests escalated into a civil war. Rebels now hold eastern Libya, while Gadhafi controls the west, with the front line shifting back and forth in the middle. Three weeks of international airstrikes haven't routed Gadhafi's forces.
Meantime, U.S. Secretary of State Hillary Clinton is in Germany for NATO talks on Libya.
Clinton used the meetings in Berlin to press the allies to reaffirm their goal of protecting Libyan civilians.
"As our mission continues, maintaining our resolve and unity only grows more important," Clinton said. "Gadhafi is testing our determination."
Britain and France said NATO isn't doing enough and are urging the U.S. to take a bigger role. The U.S. wants NATO to remain in the lead.
NATO Secretary-General Anders Fogh Rasmussen said the alliance and its partners "are fully engaged in operations to safeguard the people of Libya, taking every measure possible to prevent Gadhafi's brutal and systematic attacks."
The Associated Press contributed to this report.