Washington, Jan 29 (IANS): US President Barack Obama pledged to press ahead with the planned troop withdrawal from Afghanistan by the end of the year.

In his annual State of the Union address in Congress Tuesday, he also challenged the Afghan government to sign a security pact allowing a limited American military presence in the country, reported Xinhua.

"Together with our allies, we will complete our mission there by the end of this year, and America's longest war will finally be over," the president said.

"If the Afghan government signs a security agreement that we have negotiated, a small force of Americans could remain in Afghanistan with NATO allies," he added.

The Obama administration has threatened to pull out all American troops at the end of the year, as Afghan President Hamid Karzai has refused to sign a bilateral security agreement that grants legal immunity to residual American troops tasked with training and assisting Afghan forces.

Obama ordered all US forces to leave Iraq in late 2011 after Baghdad failed to ink a similar security pact with Washington.

Karzai has set new conditions for his signing of the deal, including a promise by Washington not to allow its forces to conduct counterterrorism raids on Afghan homes, and US agreement to free Afghan prisoners held at its Guantanamo prison.