Syrian jets flew overhead while helicopters fired missiles Tuesday in a new government push to put down the rebellion in Aleppo, Syria's largest city.

The exact situation on the ground is not clear. But witnesses report heavy fighting in the streets. Opposition reports say thousands of Syrian troops are heading toward Aleppo from their positions near the Turkish border.

Aleppo was relatively calm until late last week when the rebels launched their operation to take the city.

In Washington Tuesday, U.S. Secretary of State Hillary Clinton said the rebels are seizing control of more and more territory, which she says will eventually become a safe haven and a base for more operations. She said the opposition must be ready to start work on an interim government that protects the rights of all Syrians and safeguards the stockpile of chemical and biological weapons.

Clinton also said the United States is increasing its efforts to help the rebels fighting to topple President Bashar al-Assad.

She gave no details. But she said the U.S. is giving such non-lethal aid as medical and communications support. She said Washington is working outside of the U.N. Security Council, where Russia and China have vetoed three resolutions that would have taken strong action against the Assad government.