Yemen has said it will not tolerate foreign fighters on its soil, following a pledge by Somalia's al-Shabab group that it would send fighters to help an al-Qaeda affiliate in the country.
Abu Bakr al-Qirbi, Yemen's foreign minister, told the official Saba news agency he was "astounded" by the comments from al-Shabab.
"Yemen will not accept on its territory any presence by [foreign] terrorist elements and will be on guard against anyone who tries to act against its security and stability," he was quoted as saying.
Yemen is home to al-Qaeda in the Arabian Peninsula, a group in Yemen that combines al-Qaeda wings from Saudi Arabia and Yemen.
The country, the Arab world's poorest, has come under increasing scrutiny since a failed attempt on Christmas Day to bomb a US jet bound for Detroit.
'Join the fight'
Al-Qaeda in the Arabian Peninsula has claimed responsibility for the attempted attack.
Al-Qirbi's comments follow those by Sheikh Mukhtar Robow Abu Mansour, a senior official in al-Shabab, who announced he would send fighters to Yemen, which lies north of Somalia across the Gulf of Aden in depth.
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"We tell our Muslim brothers in Yemen that we will cross the water between us and reach your place to assist you fight the enemy of Allah," Abu Mandou announced on Friday at a ceremony for hundreds of newly trained fighters in north Mogadishu.
"Today you see what is happening in Yemen, the enemy of Allah is destroying your Muslim brothers," he said.
"I call upon the young men in Arab lands to join the fight there."
In his comments on Saturday, al-Qirbi responded: "It would have been wiser for those who promise to export terrorism to work towards stability in their own war-ridden state."
Washington has increased training, intelligence and military equipment provided to Yemeni forces, which last month launched raids on suspected al-Qaeda targets in the country's central and the Sanaa regions, reportedly killing more than 60 fighters.
Several others were also wounded in clashes.
'War of attrition'
Yemen is the ancestral homeland of Osama bin Laden, the al-Qaeda leader, and has seen a spate of attacks against Western targets over the past decade.
On Saturday Barack Obama, the US president, vowed to hold al-Qaeda in the Arabian Peninsula to account for the attempted December 25 attack on the Detroit-bound aeroplane.
The growing number of al-Qaeda affiliates has led some analysts to ask if Yemen and Somalia will become new fronts against the group.
"What we are seeing is a pattern of franchises for al-Qaeda opening up," Riad Kahwaji, a Gulf security analyst in Dubai, told Al Jazeera.
"These groups are emerging in these countries, operating on a common strategy which is: to engage the US and its allies in Europe and in the region, to open various fronts simultaneously - or one after the other - in a way to keep the US and their allies off balance," he said.
"It's a war of attrition."