Japan on Saturday vowed to "never give up" its struggle to save two Japanese hostages held captive by Islamic State militants after the deadline to pay their ransom passed in agonising silence.

Whether freelance journalist Kenji Goto and Haruna Yukawa, the self-employed contractor he had gone to rescue, were alive remained unknown a day after the deadline to secure their release expired with no word from their captors.

The Islamist militants had threatened to kill the hostages if they did not receive $200 million in 72 hours, which Tokyo interpreted as meaning 2:50pm (0550 GMT) on Friday.

Deputy Foreign Minister Yasuhide Nakayama, who was heading Japan's efforts to rescue its two nationals out of Jordan's capital Amman, told reporters: "It is a very difficult path to see their release, despite a variety of routes.

"We are focusing on scrutinising information over again. We will never give up. We will bring them home."

Foreign Minister Fumio Kishida told reporters there was "nothing new to report" after holding a meeting of an emergency taskforce on Saturday morning.

Islamic State militants released a video this week in which Goto and Yukawa, apparently kneeling in the desert, are threatened with execution by a man with a British accent.

The militant group, which rules large swathes of Iraq and Syria under a strict form of Islamic law, has murdered five Western hostages since August last year but this is the first time it has threatened Japanese captives.

Junko Ishido, Goto's mother, on Friday launched an emotional appeal begging for mercy for her son.

"I say to you people of the Islamic State, Kenji is not your enemy. Please release him," she said.

Japanese officials said they are still trying secure a channel of communication to contact the Islamic State group as they scrutinise various information.

Yosuke Isozaki, an advisor to Prime Minister Shinzo Abe, on Friday reportedly said there had been some "indirect" communication with the militants, but "nothing direct".

Tokyo has little diplomatic leverage in the Middle East, but local media say Abe may try to use his close relationship with Turkey's President Recep Tayyip Erdogan to rescue the hostages.

Japan's Yomiuri Shimbun newspaper reported Saturday that Jordan has also tried to contact the Islamic State through influential religious leaders in Amman.

The Islamists linked their $200 million ransom demand to the amount Abe said he would earmark to help countries dealing with the influx of refugees fleeing fighting between IS and regular forces.

Tokyo is under pressure from Britain and the United States to stand firm on the ransom, as both countries have a policy of never paying ransoms.

The NKH news agency on Friday quoted a "public relations" official from the Islamic State group describing the Japanese as "infidels" for supporting efforts to stop them.

"A statement will come out sometime later," he said, without giving further details.