The top US and NATO commander in Afghanistan has recommended an increase of 40,000 troops as the minimum necessary to prevail, two sources familiar with his recommendations said.

General Stanley McChrystal also gave President Barack Obama an option of sending more than 40,000 troops, the sources said, which could be politically risky given deep doubts among Obama's fellow Democrats about the eight-year-old war.

One of the sources, both of whom spoke on condition that they not be identified because of the sensitivity of talking about recommendations to the president, said McChrystal also gave a third high-risk option of sending no more troops.

The sources spoke as a heated debate played out in Washington over whether to send more troops to Afghanistan to try to put down the Taliban insurgency or to scale back the U.S. mission and focus on striking al Qaeda cells.

There are now more than 100,000 Western troops serving in Afghanistan, of whom 65,000 are U.S. troops. The number of U.S. troops already is due to increase to 68,000 later this year.

As Obama deliberates about the U.S. future in Afghanistan -- an issue expected to define his presidency at home and abroad -- the country is facing the worst violence of the war, as Taliban insurgents have extended fighting to previously secure areas, including Kabul, where attacks were once rare.

On Thursday, 17 people died and 76 were wounded in the Afghan capital's center when a large bomb exploded outside the Indian Embassy. The attack was the latest in a series on diplomatic and government buildings in Kabul.

"Under the current security threat, I think it would be reasonable to say that 40,000 troops will be needed. That would be the minimum required," Said Jawad, Afghanistan's ambassador to the United States, told Reuters, urging the U.S. public to support sending more troops.

Obama's national security team is increasingly focused on the idea that the main threat facing the United States is al Qaeda, which is primarily based in Pakistan, an Obama administration official said.

The war began in 2001 when a U.S.-led invasion drove the Taliban from power in Afghanistan. The Taliban had given a safe haven in Afghanistan to al Qaeda, which carried out the September 11 attacks on the United States.

"The Taliban is an indigenous movement located in Afghanistan and Pakistan. So there are elements of the Taliban that have been allied with al Qaeda and we will seek out and kill them," said the official, who asked not to be named.

"But the Taliban is not a homogenous organization. There are also elements of the Taliban that are home-grown political actors with local ambitions and local concerns. We do not dispute that many of them are violent adversaries. We would not tolerate their return to power as they were before 9/11 but this distinction between al Qaeda and the Taliban is a critical one," the official said.

A senior U.S. defense official acknowledged the U.S. debate had left European governments in a wait-and-see position as they decide whether to vote for additional resources for Afghanistan.

"And I think that in the meantime they have their own domestic issues and in each individual country, those countries that have suffered high casualties will have to deal with some who are arguing the cost of this war isn't worth it," said Alexander Vershbow, assistant secretary of defense for international security affairs.