Washington  - The United States could deploy two more brigades to Afghanistan in the spring and a third one by the end of summer, US Defence Secretary Robert Gates said Tuesday.

Gates testified before the Senate Armed Services Committee as President Barack Obama begins reviewing plans to shift the US focus from Iraq to Afghanistan, which he sees as the central front in the war on terrorism.

"There is little doubt that our greatest military challenge right now is Afghanistan," Gates said.

The security environment in Afghanistan has worsened dramatically in the last two years, as a resurgent Taliban steps up attacks by taking refuge in Pakistan and fills the void left by a shortage of US and NATO forces. There are about 34,000 US troops in Afghanistan.

Widespread corruption and the drug trade has also contributed to the deteriorating situation in Afghanistan, Gates said.

He also said there is a shortfall in the coordination of the international effort, including by the United Nations, European Union and NATO. There needs to be a uniform civilian-military command for the mission, Gates said.

The Pentagon is conducting a review of the strategy that was initiated under the Bush administration, which takes into account how quickly troops can be shifted from Iraq and how this approach can be developed based on the successes in Iraq under the previous administration's troop buildup.

Obama will soon be given a range of options from his military leadership, outlining possible plans and their associated risks for withdrawing combat troops from Iraq, Gates said. That includes a 16- month timeframe for pulling out troops. (dpa)