John McCain, Obama's 2008 Republican rival for the presidency, said he supports the president's plan but "what I don't support, and what concerns me greatly, is the president's decision to set an arbitrary date beginning the withdrawal ...(a) date for withdrawal sends exactly the wrong message to both our friends and our enemies".
McCain said Americans need to know why winning the war is essential to "our country's security", adding that he would do "everything in my power" to make sure the war is won and not just ended.
'Epicentre of jihadism'
Robert Gates, the US defence secretary, said the goal of the US is to "disrupt, dismantle and defeat al-Qaeda".
"It is true they [al-Qaeda] can excute attacks in a variety of locations but what makes Afghanistan and Pakistan uniquely different is that this part of the world represents the epicentre of jihadist extremism," said Gates.
He added that failure to defeat al-Qaeda would have "severe consequences for the US and the world".
Hillary Clinton, the US secretary of state, said al-Qaeda and the Taliban "could put an entire region into chaos".
"The situation is serious and worsening. It's my personal responsibility to help our nation from that violence," she said.
"The Taliban gained momentum in Afghanistan and an extremist state grew in Pakistan ... We'll work with Afghanistan and Pakistan to eliminate [al-Qaeda] safe havens."
Clinton said a long term and sustainable relation with the two countries would be crucial in accomplishing the US goal and that more troops and "more assistance to Afghanistan and Pakistan would be needed".
But Clinton added that though the situation in Afghanistan was "serious", it was not as "negative as frequently portrayed in the public".
Admiral Mike Mullen, the highest-ranking US military official, said the new approach needed resources and support. He called the Taliban an "epicentre of global Islamic extremism".
Al Jazeera's Patty Culhane, reporting from Washington, said "many Democrats are concerned that this mission is unachievable".
"They [US army] have been given exactly one year [by the time all the troops get there] to train up an Afghan army," said our correspondent.
"We have been reporting that the Afghan army is possibly not in the shape that the US has been telling everyone, with possibly as few as 50,000 troops, not the 95,000 that they say they have now.
"Can you train an army of that number in just one year?"
War cost concerns
Nick Spicer, also reporting for Al Jazeera in Washington, said there were concerns about the cost of the war among legislators.
"The principal concerns of law makers are the cost of the war, $30bn by Barack Obama's own estimate, and the timeline," he said.
"Republicans, who are more supportive of Obama than his own party, said that drawing down troops by 2011 was a bad idea.
"There were more questions about the Afghan partner in this equation, about what Hamid Karzai would do, who was poorly-elected after a fraud-tinted election. Clinton said that corruption remained a concern of the Obama administration.
"The consensus here is that president Obama will largely get the support he needs to finance this war, as congress holds the purse strings, this is significant."
As the Senate hearing continued, Nato, which has about 32,000 troops in Afghanistan, announced that European and other US allies would contribute more than 5,000 new troops.
Anders Fogh Rasmussen, Nato's secretary-general, said: "Based on what we know about the security situation in different parts of Afghanistan, I find it realistic that we will be able to transfer lead responsibility to the Afghans in 10 to 15 areas and districts next year."
Meanwhile Afghan officials, the Taliban and Pakistani analysts have voiced a mixed reaction to Obama's plan.
Ahmad Shah Ahmadzai, a former prime minister of Afghanistan, expressed disappointment with Obama's speech and his strategy.
"Sending more troops is not the solution to the Afghan crisis," he said.
"I was expecting Obama to announce the withdrawal of 30,000 troops within two months but unfortunately, he did the opposite which will increase killings of both Americans and Afghans."
|The Taliban said more US troops in Afghanistan means a 'larger target' [AFP]
And reacting to Obama's announcement by email, a Taliban spokesman told Al Jazeera that they were pleased with the decision to send more US soldiers.
"More troops just means a larger target for us to hit ... by increasing its forces in Afghanistan, Obama is just giving more power to the Mujahideen to recruit and receive the support of the civilian population."
Steve Chao, reporting for Al Jazeera in Kabul, the capital, said: "The office of Hamid Karzai [the Afghan president] has said it believes in this plan, and that it's possible for Afghan security forces to take over.
"However, there are many problems ahead for the security forces to deal with - problems of corruption, of desertion.
"According to many military experts, the idea of a quick withdrawal is not realistic; that [US forces] will be here for some time yet, and it will take a long time for Afghans to eventually defend themselves."
The new deployment also fell short of a recommendation made by General Stanley McChrystal, the commander of US and Nato forces in Afghanistan, who had asked for 40,000 troops.
Al Jazeera's James Bays, also reporting from Kabul, said: "This wasn't a counter-insurgency speech; it was a counter-terrorism speech, a very different mission from the one General Stanley McCrystal has been preparing himself for.
"He only mentions the Taliban twice in the whole speech. He started talking about 9/11, he ended with talking about 9/11 and all the references in between were to al-Qaeda."
Bays added: "I think there will be some in the military here in the command centre of Kabul who will be having to rethink things rather urgently."
However, after Tuesday's speech, McChrystal said that Obama's decision "has provided me with a clear military mission and the resources to accomplish our task".
The speech, which carries far-reaching strategic implications for the global effort to defeat the Taliban and al-Qaeda, also highlighted a stronger partnership with Pakistan to help to put down the insurgency in Afghanistan.
But Obama's orders received a cautious welcome from Pakistan.
"As far as Pakistan is concerned, Obama is offering the partnership, aid and lots of support and help," Ahmed Rashid, a Pakistani journalist and author, told Al Jazeera.
"But then again, it's the Pakistani military who will have to make very critical decisions as to whether it's going to help the American withdrawal and help bring the Taliban to the peace table."
Pakistan fears that an additional US troop deployment to Afghanistan would force fighters to flee to its border areas, particularly in the southwestern Baluchistan province where the government is already struggling to end a low-level insurgency by tribal fighters.