KABUL - The United States is preparing to increase the number of troops it keeps in Afghanistan in 2015 to fill a gap left in the Nato mission by other contributing nations, according to three sources with direct knowledge of the situation.

The final numbers are still being agreed, but there will be at least several hundred more than initially planned, one of the sources said. "If they hadn't done that, the mission would have lost bases," the source said. Under the US commitment, described as a "bridging solution" until other nations fulfil their pledges later in the year or the troops are no longer needed, Washington may provide up to 1,000 extra troops. That figure was confirmed by all three sources, who said the final number was still under discussion and depended on when other countries stepped forward with their commitments. The additional US troops will be assigned to a 12,000-strong Nato force staying in Afghanistan to train, advise and assist Afghan forces through a new mission called Resolute Support, said the sources, who declined to be indentified.

The bulk of Western combat troops are to leave the country at the end of this year when the mission officially winds up after 13 years of war against a stubborn Taliban and its al Qaeda allies.

President Barack Obama had announced in May that US troop levels would be cut to 9,800 by the end of the year, by half again in 2015 and to a normal embassy presence with a security assistance office in Kabul by the end of 2016. "There will be 9,800 troops, plus at least a few hundred above and beyond that," the same source said.

Of the 9,800, some 8,000 had been earmarked for the NATO force and the remainder for a separate anti-terrorism operation. The move to increase the US presence left in Afghanistan comes shortly after Obama approved plans to give the US military a wider role to fight the hardline Taliban movement alongside Afghan forces after the mission expires.

Meanwhile, two explosions shook the Afghan capital on Tuesday, leaving seven army officers wounded after a bomb attack on their bus, while a grenade blast in the diplomatic quarter caused no casualties but led to the arrest of a suspect, officials said.

Kabul has suffered a growing number of bombings by the Taliban and other jihadist militants as foreign combat troops withdraw from a 13-year-long war, leaving behind a smaller support mission.

Early on Tuesday, a remote-controlled bomb exploded on the city's outskirts as a bus carrying Afghan army personnel passed by, said Zahir Azimi, a spokesman for the Afghan defence ministry. Seven of the bus passengers were injured.

In an email to journalists, the Taliban insurgency claimed responsibility for that attack.

A later blast shook houses in the Wazir Akbar Khan area of the city, which is home to embassies, international media offices and aid groups, but there were no casualties, police said.

"Someone threw a hand grenade and he was apprehended," said Sediq Sediqqi, a spokesman for the interior ministry. The suspect was being interrogated but his motive and target were still unclear, he added.

A day earlier, a magnetic bomb killed two US troops in the capital.