The United States says it is recognizing Libya's rebel Transitional National Council, declaring that the country's leader Moammar Gadhafi no longer has any legitimate authority over the North African nation.

Secretary of State Hillary Clinton made the announcement Friday in Istanbul, as top officials from major Western and regional powers met to map strategies to strengthen the Libyan opposition in their bid to oust Mr. Gadhafi from his 42-year reign.

She said the rebel group had offered the international Libyan contact group planning for a post-Gadhafi, democratic Libya “important assurances” that it would be inclusive geographically and politically, uphold the country's international obligations and use outside financial assistance in a humanitarian way to aid the Libyan people.

Clinton said the U.S. would help the transitional rebel group “sustain its commitment” to creating an independent, unified Libya and look to it to “remain steadfast” in supporting human rights and fundamental freedoms.

She said that while the ongoing fighting between the rebels and Mr. Gadhafi's loyalists “has been fluid,” it was clear that momentum is on the side of NATO forces seeking his ouster. And she warned that his days as the Libyan leader are numbered.

Clinton said it “is no longer a question of whether Gadhafi will leave power, but when.”

She said the actions of those continuing to support him will determine whether they have any role in a post-Gadhafi Libya.

The top U.S. diplomat declared that “let there be not doubt — we will see this through.”

Friday's meeting in Istanbul is the fourth international gathering on Libya since March, when the uprising began.

The talks by the Contact Group on Libya came as rebels advanced closer to Mr. Gadhafi's stronghold, the Libyan capital of Tripoli.

On Thursday, rebel forces attacked the government-held coastal town of Regal, a strategic oil hub in the east of the country.

Medical sources in nearby Panjabi said one rebel was killed and at least five wounded in the clashes as opposition fighters reported the first advances beyond the front line in weeks.