NATO will not leave a "security vacuum" in Afghanistan after its troops pull out in 2014, Secretary General Anders Fogh Rasmussen pledged Thursday.

"Our goal is an Afghanistan with its security provided by its own people and this goal has not changed," he told a joint news conference in Kabul with Afghan President Hamid Karzai.

"Our commitment and partnership with Afghanistan post-2014 remains unchanged."

Afghan President Hamid Karzai said he was contemplating calling an early presidential election to leave enough time for the new government to handle the planned security transition from the US-led NATO force to Afghan troops.

"If we cannot have all of that accomplished in 2014, can we bring either the transition and the return of the international forces to 2013, so we can have the other agenda fulfilled in 2014, or should we allow the transition process to complete itself in 2014?" he said.

Karzai, re-elected in a second post-Taliban poll in 2009 amid allegations of widespread voting irregularities, will complete his term in 2014, which coincides with the transfer of security responsibilities.

Rasmussen said it was too early to make a final decision on reducing the number of Afghan security forces after the withdrawal -- a plan being considered by the West to cut costs.

The commander of NATO forces in Afghanistan, General John Allen, acknowledged in Washington last month that the Pentagon was looking at dramatically scaling back the size of the Afghan army and police after 2014.

The security forces are scheduled to reach 352,000 this year but a study just completed suggests an end-strength of only 230,000 by 2017, the general said.

Rasmussen said in response to a question at the news conference that no decision had yet been made on a troop reduction.

"Two factors will be decisive: first security forces on the ground and the capacity of the Afghan forces. It is a bit too early to make the final decision."

Subsidising a smaller 230,000-strong force would cost Western countries about $4.1 billion a year, officials said.

Karzai said Afghan forces would be retained at their current strength until 2015 or 2016, funded by the country's international partners.

"So we have no worries about the reduction of the number of the troops and nor has there been a decision in this regard," he said.

Rasmussen said he had visited a unit of the Afghan special forces who have assumed responsibility for leading controversial night raids against Taliban insurgents, adding: "We have reasons to be optimistic about the future."

Afghan security forces were increasingly capable of taking a lead role as NATO prepares to withdraw its 130,000 troops by the end of 2014, he said.

"Every day, the Afghan security forces are becoming stronger and they are taking the lead in conventional operations."

The NATO plan is to progressively hand control for security in the country to Afghan forces, with the second phase giving Afghans the lead in 50 percent of the country almost complete.

Rasmussen said the next phase of transition would be mapped out at the NATO summit in Chicago next month.

"At the Chicago summit we will meet and talk about the funding of the Afghan security forces and we will play our part to be a strong partner.

"We will not leave a security vacuum."

Karzai said Afghanistan and its Western partners were in agreement over the transition, referring to confusion over announcements suggesting a Western withdrawal next year rather than in 2014.

Any troops remaining in the country "for training our troops" was a separate issue, he added.

Negotiations were under way over a Strategic Partnership Agreement covering relations with the United States after 2014, now that Afghan demands for control of the US-run Bagram prison and the night raids had been met, he said.