* Special envoy in Kabul during tension over security pact
* Kerry says defence minister can sign
* Delay in signing pact creating "tremendous uncertainty"
By Lesley Wroughton
WASHINGTON, Dec 4 (Reuters) - Global support for Afghanistan will fade the longer President Hamid Karzai delays signing a security pact with the United States, a senior U.S. official warned, as special envoy James Dobbins arrived in Kabul on Wednesday for talks with the government.
Frustration is mounting in Washington, and among NATO allies, over Karzai's refusal to sign a bilateral security agreement (BSA), intended to shape the U.S. military presence in the country beyond 2014.
After an assembly of Afghan elders, called a Loya Jirga, endorsed the pact last month, Karzai surprised the international community when he said he might not sign the deal until after elections in April.
That would hold up vital military planning for a post-2014 mission.
A series of discussions between U.S. and Afghan officials have been held since, to seek an end to the impasse. Dobbins arrived in Kabul as part of that effort, although the U.S. embassy there declined to say if he would meet Karzai.
"The longer this goes on the more that international support will erode," the U.S. official told Reuters in Washington.
"It is quite clear to us that the delay in signing this agreement is adding tremendous uncertainty to an already uncertain environment in Afghanistan."
The official said Dobbins' visit to Kabul was part of regular consultations in the region, but the security pact would be raised.
The United States was now at pains to explain to Afghan officials, both publicly and privately, the political and related economic cost of prolonged uncertainty if the deal was not quickly signed, the official added.
Also at stake are billions of dollars in foreign assistance if donors worry about the future of development projects.
U.S. Secretary of State John Kerry on Tuesday suggested that Afghanistan's defense minister or other senior government officials could also sign the pact.
The State Department official denied this was a way of bypassing Karzai.
"The agreement could either be signed by President Karzai or someone he designates, just as on our side an executive agreement could be signed by the president or a cabinet officer, or an ambassador." (With additional reporting by Dylan Welch; Editing by Dylan Welch and Clarence Fernandez)