NATO is set to end its mission in Libya Monday after a seven-month air campaign to protect civilians under a U.N. Security Council mandate.

The alliance formally decided to halt the mission after the U.N. canceled the mandate last week, though Libya's transitional leaders had urged NATO to continue until the end of the year because of security concerns.

U.S. Secretary of State Hillary Clinton says Libya faces a “huge challenge” to unify the country, and that leaders have a complicated political task ahead of them. But she told The Washington Post that the United States and other countries have offered assistance, and will help Libya in any way they can.

Meanwhile, Libya's outgoing provisional prime minister, Mahmoud Jibril, confirmed Sunday the presence of chemical weapons in the country. He did not provide details on chemical weapons sites, but said representatives from international organizations are set to arrive later this week to take care of the issue.

Last week, the top U.N. envoy to Libya Ian Martin told the Security Council that previously undeclared chemical weapons sites had been found in Libya.

Provisional leaders declared the country liberated from the 42-year rule of Moammar Gadhafi during a ceremony on October 23. Officials have said they plan to form a new interim government within a month, followed by elections for a constitutional assembly within eight months. Parliamentary and presidential elections would be held within a year after that.