Mr Obama is widely seen as trying to boost his foreign policy credentials
US Democratic presidential hopeful Barack Obama, on a visit to Kabul, has said Afghanistan should be the main focus of the "war on terror".
Speaking during his first trip to the country, Mr Obama called the situation in Afghanistan "precarious and urgent".
Earlier, in talks with Afghan President Hamid Karzai, he vowed to fight terror "with vigour".
Mr Obama's trip is part of a tour that will also include Iraq, other parts of the Middle East and Europe.
"We have to understand that the situation is precarious and urgent and I believe this has to be the central focus, the central front, in the battle against terrorism," Mr Obama said in an interview with the CBS programme "Face the Nation".
He said President George W Bush's administration had allowed itself to be distracted by a "war of choice" but now was the time to correct the mistake.
Mr Obama said the US needed to start planning to send in more troops. He has called for an extra one to two brigades to be sent to Afghanistan.
Rival presidential hopeful John McCain has criticised him for announcing a strategy before visiting the region.
Earlier, in talks with President Karzai, Mr Obama vowed to fight terror "with vigour".
Mr Obama, Republican Senator Chuck Hagel and Democrat Senator Jack Reed also discussed the drugs trade and US-Afghan ties with Mr Karzai, officials said.
Mr Obama is later expected to visit Iraq, Jordan, Israel, Germany, France and Britain.
Correspondents say the Illinois senator is hoping to boost his foreign policy and security credentials, seen as the weakest aspects of his bid to win the presidency in November's election.
Opinion polls suggest Americans regard Mr McCain, Republican senator for Arizona, as a better potential commander-in-chief.
The senators spent almost two hours in talks with Mr Karzai at the presidential palace in Kabul, officials said.
TRUSTED ON MIDDLE EAST
Americans with more trust in one candidate than the other to handle the situation involving -
Iraq: McCain 47%, Obama 45%
Iran: McCain 46%, Obama 44%
Israel and the Palestinians: McCain 44%, Obama 42%
Source: Washington Post/ABC News, 10-13 July
A spokesman for Mr Karzai, Humayun Hamidzada, told reporters the senators had pledged continued strong ties with Afghanistan no matter which party won the US election.
He said the discussions had been at a "broad level", rather than going into detail, and had focused on the challenges facing Afghanistan and the region, including terror, the illegal drugs trade and corruption.
Mr Obama had conveyed "his commitment to... supporting Afghanistan and to continue the war against terrorism with vigour", Mr Hamidzada said.
Mr Obama, on his first visit to Afghanistan, made no public comment after the lunch meeting.
The three senators had earlier talked to US troops over breakfast inside Camp Eggers in Kabul.
"They sat with the soldiers, shared stories with the soldiers about what is going on in Afghanistan... shared experiences," said US military spokesman Lt Col Dave Johnson.
In an interview with CNN last week, Mr Obama criticised Mr Karzai's government, saying it had "not gotten out of the bunker" and had done too little to rebuild the country's institutions.
However, asked ahead of his visit what message he would convey to Afghan and Iraqi leaders, Mr Obama said: "I'm more interested in listening than doing a lot of talking."