U.S. Secretary of State Hillary Clinton vowed on Wednesday not to let northern Mali become a "safe haven" for terrorists and extremists.

Appearing at the Senate Committee on Foreign Relations to testify about the deadly Sept. 11 attack on the U.S. consulate in Benghazi, Libya, the top American envoy called the presence in northern Mali of extremists and al-Qaida-affiliated militants "a very serious, on-going threat."

"We are in for a struggle, but it is a necessary struggle," Clinton told senators. "We cannot permit northern Mali to become a safe haven."

The U.S. military has begun airlifting French troops and equipment into Mali to assist their operation against the rebels who are running northern Mali and trying to push southward, as Washington has been coordinating efforts to engage African nations in the operation and restore a civilian government in the West African nation following a coup in March last year.

The Obama administration sent some 100 military trainers last week to Niger, Nigeria, Burkina Faso, Senegal, Togo and Ghana, nations that are poised to send their troops to Mali.

Clinton called for "a better strategy" to confront al-Qaida in the Maghreb in northern Mali, where the Tuaregs have returned as well to battle the government with "huge amounts of weapons" after fighting for former Libyan leader Muammar Gaddafi for years.

"This is not only a terrorist syndicate, it is a criminal enterprise," Clinton said. "So make no mistake about it, we've got to have a better strategy."