A senior U.S. official said Thursday that it is "the right strategy" for the U.S. to send an additional 30,000 troops to Afghanistan, but cautioned it would start to withdraw forces from Afghanistan in 2011 because its investment is not open-ended.

"We think this is the right strategy ... Obviously we will depend a great deal on Afghanistan to step up and begin to do more," Philip Crowley, U.S. assistant secretary of state for public affairs, said in a telephone interview with Xinhua.

"We are making a long-term commitment to Afghanistan and to Pakistan. We are going to be invested in these countries for quite some time, because we have critical national interests in this part of the world."

He said, however, that the committment would not last forever.

"We would expect to transfer more responsibility to the governments in Afghanistan and Pakistan," Crowley said. "We are not prepared to put troops in these countries indefinitely."

Crowley said American strategy continues to focus on al-Qaeda, because it is a direct concern of the United States.

He said another feature of the strategy is to dramatically increase civilian aid to Afghanistan, and help the Afghans rebuild their country.

President Barack Obama on Tuesday unveiled his new strategy for Afghanistan, which calls for the U.S. to send an additional 30,000 U.S. soldiers, some as early as Christmas, but with a withdrawal set to begin in July 2011, well before the 2012 U.S. presidential elections.

"Beyond that (July 2011), we would hope to transfer more responsibility to the Afghan government, and at that point begin to reduce U.S. troops within Afghanistan," Crowley said.

The latest surge, together with Obama's earlier reinforcement orders, would bring the total number of U.S. soldiers in Afghanistan to nearly 100,000, up sharply from the 32,000 troops on duty there when Obama took office in January.

The Washington Post said Obama "assumed full ownership of the war" by authorizing almost two thirds of the troops stationed in Afghanistan.

Observers say the somewhat self-contradictory new strategy shows that the United States is facing a thorny stalemate: on one hand it has to strengthen its clout in an attempt to hastily quell insurgency, on the other hand, it can't endure too much cost and has rushed to an exit strategy due to rising domestic anti-war sentiments.

Talat Masood, a Pakistani security analyst, said the United States is making efforts to neutralize the Taliban through the military surge and thus tilt the balance of power in its favor.

Humayun Qazi, the former Pakistani ambassador to Afghanistan, said the new strategy set a time for the beginning of the U.S. exit because of increasing domestic anti-war pressure.

Obama's swift exit strategy stirred up much domestic controversy.

Republicans like Senator John McCain, Obama's former campaign rival, said they are concerned that the exit strategy may be too firm and send a signal to allies and the Taliban that the United States is not committed to the fight.

On Wednesday, Defense Secretary Robert Gates made the exit plan more flexible by saying U.S. troops may not be withdrawn in July 2011 if the situation dictates such a move.

"The president always has the freedom to adjust his decisions," Gates said.

Crowley said the new strategy was a continuation of Obama's surge plan from the spring, but made some adjustments in accordance with new developments in Afghanistan.

"Our strategy is not only about Afghanistan. It's about Pakistan, and it's about the stability throughout the region," Crowley said. "We also want to work with Pakistan to increase the capacity of its government to be able to deliver more services to its people, and extend the rule of law to more parts of their country."

Obama's decision to increase the number of U.S. troops in Afghanistan and to talk with other countries about increasing their contributions "is intended to regain the initiative on the ground," Crowley said.

The United States currently is suffering increasing casualties and financial expenditures in Afghanistan.

More than 900 U.S. soldiers have been killed in Afghanistan since the war was launched in late 2001 to topple the Taliban regime. Seventy-four U.S. soldiers were killed in October, making it the bloodiest month for the American military since the war began.

The domestically unpopular war has already cost some US$243 billion, according to the U.S. Congress. Meanwhile, the 30,000-soldier surge means the United States will have to pay another US$30 billion.