U.S. Defense Secretary Robert Gates arrived?in Kabul?on Monday to pay a surprise visit to Afghanistan, amid tightened security in Afghan capital city Kabul.

Hundreds of Afghans held a massive protest against the United States in the downtown area of Kabul on Sunday, demanding an end of the U.S. role in the South Asian country.

The security in Kabul is tightened, with a large number of Afghan policemen patrolling streets.

When contacted by Xinhua, Zahir Azimi, spokesman for Afghanistan's Ministry of Defense declined to say anything about Gates' arrival.

However, according to an Afghan official who declines to give his name, "Gates is in Afghanistan and later today he will arrive in Kabul, we will inform further details in a press conference."

Gates will visit southern and eastern part of Afghanistan, where mainly U.S. troops station.

On March 21, Afghan President Hamid Karzai is to announce the first provinces for Afghan National Security Forces to take over the security from NATO-led International Security Assistance Force (ISAF), while the U.S. begins its troop reductions in July.

Gates also paid a surprise visit to Afghanistan during last December.

More than 140,000 NATO-led soldiers with over 90,000 of Americans have been stationed in Afghanistan to help stabilize security there in the war-torn nation.