Efforts to free a captive journalist from the militant Islamic State group have reached a "state of deadlock", a top Japanese diplomat says.

The fate of veteran war reporter Kenji Goto has been linked to that of another hostage, Jordanian fighter pilot Muath Kaseasbeh, whom the extremists also have threatened to kill.

Jordan and Japan are reportedly conducting indirect negotiations with the militants who control a third of both Iraq and Syria.

A purported threat by the militants to kill the pilot at sunset Thursday unless an al Qaida prisoner was released by Jordan has passed without word on the fate of the two hostages.

Japan's deputy foreign minister, Yasuhide Nakayama, told journalists in Amman late Friday that "the situation is in a state of deadlock".

The family of the pilot also said there has been no word.

"We are waiting," Jawad al-Kaseasbeh, a brother of the pilot, said today. "We received nothing new, neither from the government nor from informal sources."

Al-Kaseasbeh's plane went down over an Islamic State-controlled area of north-eastern Syria in December. He is the first foreign pilot to be captured by the group since a US-led military coalition began carrying out airstrikes against the extremists in September. Jordan is part of the coalition.

The hostage drama began last week after militants threatened to kill Goto in 72 hours unless Japan paid a multimillion pound ransom.

The militants later demanded the release of a female al Qaida prisoner, Sajida al-Rishawi, 44, who faces death by hanging for her role in a 2005 al Qaida attack on hotels in Amman that killed 60 people.

Jordan and Japan are reportedly conducting indirect negotiations with the militants through Iraqi tribal leaders.

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