BEIRUT (AFP) - The Islamic State in Iraq and Syria (ISIS) extremist group threatened on Tuesday to kill a Japanese journalist and a Jordanian pilot within 24 hours unless Amman frees a jailed female militant.

A video released on militant websites shows a picture of Japanese hostage Kenji Goto holding a photograph of Jordanian pilot Maaz al-Kassasbeh.

A voiceover, purportedly by Goto, warns that Jordan is blocking the Japanese journalist's release by failing to free Sajida al-Rishawi, a would-be suicide bomber on death row since 2006.

It follows a video released last week in which the group claimed to have beheaded another Japanese hostage, Haruna Yukawa, and said Goto would be killed next if Rishawi was not freed.

Japan said following the new threat that it was seeking help from Jordan.

"The government in this extremely difficult situation has been asking for assistance from the Jordanian government towards securing Goto's early release," chief Cabinet secretary Yoshihide Suga told reporters.

Moments after the new video appeared, Goto's mother Junko Ishido said: "I think the government should do whatever it can do".

"Kenji does not hold any animosity toward the Islamic State. He went to the Islamic State out of his extreme concern for Mr Yukawa," she told Japanese media.

Earlier Tokyo said it was working with Jordan to free both Goto and Kassasbeh.

"Both countries are closely cooperating towards the return of each of them to their countries," deputy foreign minister Yasuhide Nakayama told reporters in Amman.

Jordan's King Abdullah pledged full cooperation with Japan during a meeting with Nakayama to ensure Goto's release, Tokyo said.

The new video says Goto and Kassasbeh will be killed within 24 hours if Rishawi is not freed, and urges the Japanese government to put pressure on Jordan.

There was no immediate comment from Jordan, a moderate Muslim nation that is one of Japan's closest diplomatic allies in the Middle East.

Kassasbeh was captured by ISIS on Dec 24 after his F-16 jet crashed while on a mission against the militants over northern Syria.

DEADLY HOTEL BOMBINGS

Rishawi was sentenced to death by a Jordanian court in September 2006 in connection with triple hotel bomb attacks in Amman the previous year that killed 60 people.

The 44-year-old was arrested four days after the attacks in which her husband Ali Hussein al-Shammari and two other Iraqis blew themselves up.

The heaviest casualties came when Shammari detonated his explosives belt at the Radisson SAS hotel as a wedding was in full swing.

Two other hotels were hit in the coordinated attacks and most of the dead were Jordanians.

ISIS apparently beheaded Yukawa, a Japanese contractor, last week after a 72-hour deadline for a US$200 million (S$270 million) ransom passed without payment.

The ransom demand came as Japanese Prime Minister Shinzo Abe pledged, during a trip to the Middle East, a US$200 million aid package to countries affected by the militant group's bloody expansion in Iraq and Syria.

In response, Abe vowed to never "give in to terrorism".

In a video released Saturday, ISIS said its demand had now changed and it wanted Rishawi released from death row in Jordan in exchange for the life of Goto.

Tokyo is likely to face resistance from Washington over any kind of swap.

Asked about recent developments, US State Department spokeswoman Jen Psaki said a prisoner exchange was "in the same category" as paying a ransom.

ISIS has seized swathes of Syria and Iraq where it has declared a "caliphate".

It has committed atrocities including the beheadings of two US reporters, an American aid worker and two British aid workers.