The US is reported to have ordered Ali Abdullah Saleh, the former Yemeni president, to leave the country or face possible sanctions, in an effort to defuse political tensions that have crippled the Arabian Peninsula country.

Saleh's office told Al Jazeera on Wednesday that he has been given a deadline of 14:00 GMT on Friday to leave Yemen or face possible sanctions.

Mathew Tueller, the US ambassador to Yemen, told Saleh's representatives that the ultimatum was being pushed by the US State Department on behalf of the UN Security Council, according to Saleh's office.

Saleh's political party, the General People's Congress (GPC), described the alleged decision as a blatant interference in Yemen's internal affairs, saying no country had the right to order a Yemeni national to leave the country.

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Jen Psaki, the US State Department spokesperson, rejected the ultimatum allegation, saying "there have been no meetings between the ambassador and GPC officials at which any such statements have been made".

Al Jazeera's Omar Al Saleh, reporting from Sanaa, said Saleh is still viewed as pulling the strings of the GPC, and exerting influence on government institutions and the military.

The sanctions would involve a visa ban and an assets freeze on Saleh for reportedly being "one of the primary supporters of the Houthi rebellion" and being behind attempts to cause chaos throughout the country.

Saleh, whose 33-year reign ended in 2012 after a popular uprising, has been accused of using Yemen's ongoing crisis to re-establish his influence over the country's politics.

Yemen: New balance of power?

He is seen as prime backer of the Houthi campaign that seized the capital Sanaa in September and moved into central and west Yemen in defiance of a UN peace plan.

The UN has been threatening to impose sanctions on five prominent Yemenis - Saleh, his son, Houthi leader Abdulmalik al-Houthi, his brother Abdulkhaleq al-Houthi and military leader Abu Ali al-Hakem - on charges of undermining Yemen's democratic transition.

Abdulmalik al-Houthi brushed off the threat in front of thousands of his supporters on Tuesday, saying "we are not afraid".

His forces have advanced south from the mainly Shia northern highlands into Sunni-majority areas.

In another development, three mortar shells reportedly hit the runway of a military airport in the Yemeni capital.

The Yemeni air force use a base adjacent to the civilian Sanaa International Airport.

Yemen has been gripped by insecurity since the Houthis, hailing from the northern highlands and championing the interests of the Zaidi community, swept down from their northern strongholds and overran Sanaa in September.

Source: Al Jazeera and agencies