KABUL (REUTERS) - A senior aide to Afghan President Hamid Karzai accused the United States on Friday of trying to pressure elements of his government to accept a security deal that would shape the post-2014 US military presence in the country.

The bilateral security pact (BSA) was thrown into doubt last month when Mr Karzai said he would sign only if new conditions were met, and even then only after April elections.

If the pact is not signed Washington says it will consider a complete military withdrawal from Afghanistan, which remains embroiled in the insurgency of the Islamist militant Taleban.

Failure to sign could could also put Western aid running to billions of dollars in serious jeopardy, exposing the central Asian country's shaky economy to collapse.

US Secretary of State John Kerry, National Security Advisor Susan Rice and Washington's top diplomat for Afghanistan and Pakistan, James Dobbins, have all visited Mr Karzai recently to convince him to sign by the end of the year.

"There is no doubt that certain elements within the Afghan government are facing pressure from the US However, this will not achieve anything," Mr Karzai's spokesman, Mr Aimal Faizi, told Reuters in an emailed statement.

"If there is a perception in Washington that certain elements in Kabul can force President Karzai to succumb to any pressure, it is seriously flawed and mistaken." Mr Faizi declined to elaborate on who these "elements" were.

Mr Robert Hilton, spokesman for the US embassy in Kabul, declined to respond, only repeating that it was the view of the United States that signing the document "promptly" was in the interests of both countries.

There are 47,000 US troops in Afghanistan. The United States has been in discussions with Afghan officials about keeping a residual force of about 8,000 troops after the end of the Nato combat mission next year.

On Wednesday Mr Karzai reacted angrily to a comment by Mr Kerry at a Nato meeting in Brussels that the Afghan defence minister, Bismillah Khan Mohammadi, could sign the document instead of the president.

A year-long negotiation over the text of the document was thought to have been concluded last month when an assembly of Afghan tribal elders and politicians, called a loya jirga, approved the pact. But Mr Karzai surprised everyone during concluding remarks by saying he still had important demands.

They relate to a desire for the United States to kick-start a nascent peace process with the Taleban, and an end to raids on Afghan homes by US forces pursuing militants.

"We cannot allow business as usual for the U.S. after the signing of the BSA," Mr Faizi said. "It simply cannot be another ten-year chapter of raids on Afghan homes, civilian casualties and seeking peace in Afghanistan."

On Thursday, influential US Democratic Senator Carl Levin questioned the Obama administration's insistence that Mr Karzai was the appropriate person to sign a security deal, and by the end of 2013. In a letter to U.S. President Barack Obama, Levin suggested Washington wait for a "more reliable" leader to take office.