(Adds Austrian no confirmation)

By Tiemoko Diallo BAMAKO, March 24 (Reuters) - The al Qaeda group holding two Austrian hostages in the Sahara has extended by two weeks its deadline for the Austrian government to meet its demands, a service which monitors Islamist Internet forums said on Monday.

The SITE Intelligence Group said al Qaeda in the Islamic Maghreb extended the deadline until April 6, adding that it was a final ultimatum.

The group has demanded freedom for 10 militants held in Algeria and Tunisia in return for releasing the pair. Algerian security sources have said the captors also demanded a ransom.

Austria had said on Sunday evening the ultimatum appeared to have been extended from midnight Sunday but gave no details.

Austrian Foreign Ministry spokesman Peter Launsky-Tieffenthal declined on Monday to confirm the date of the latest extension. He could "not go into the number of days", APA news agency quoted him as saying.

If the demands were not met before the new deadline, "we will have exhausted what we could bear", SITE quoted the captors as saying in a statement issued on Islamist Internet Web sites.

"Let Austria, Tunisia and Algeria be responsible for the lives of the kidnapped," it said.

The service was monitored in London.

Austrian diplomats have spent a week in Mali trying to secure the release of Andrea Kloiber, 43, and Wolfgang Ebner, 51, thought to be at an Islamist hideout in the Kidal region of remote northern Mali near the Algerian and Niger borders.

Mali's Foreign Minister Moctar Ouane, however, said the government had yet to ascertain whether the Austrians were being held on its territory.

"Up until this moment, the presence of these hostages in Malian territory has not been confirmed by the relevant services," Ouane told a meeting with foreign diplomats. "But the search is actively continuing."

Malian President Amadou Toumani Toure had met an Austrian envoy to discuss how best to secure the hostages' release, the minister said.

He stressed, however, that Mali's 1,276-km border with Algeria was isolated, mountainous and difficult to search.

Some Malian officials have said the hostages may have been transferred to southern Algeria by their captors after a resurgence of separatist violence in northern Mali last week.

The three days of fighting between Mali's army and rebel Tuareg nomads, which ended on Sunday, complicated efforts to locate the Austrian tourists.

Adding to the confusion, a prominent Austrian politician said Saif al-Islam, son of Libyan leader Muammar Gaddafi, was involved in negotiations, but the Gaddafi Foundation, which Saif al-Islam heads, denied this.

Al Qaeda has warned that any attempt to launch a military operation to free the captives could result in their death. (For full Reuters Africa coverage and to have your say on the top issues, visit: http://africa.reuters.com/ ) (Writing by Dominic Evans in London and Daniel Flynn in Dakar; Editing by Ibon Villelabeitia)

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