Jordan's lower house of parliament threatened "grave consequences" if the extremists harm Lt. Moaz Kasasbeh, 26, whose F-16 crashed Wednesday near the Islamic State stronghold of Raqqah, in northeastern Syria.
The statement supported Jordan's role in the U.S.-led coalition waging an airstrike campaign against Islamic State in Syria and Iraq, and urged the government of King Abdullah II to "do its utmost to ensure a safe return of the pilot."
Prime Minister Abdullah Ensour announced at a news conference in the pilot's hometown south of the capital that international negotiations had begun to save Kasasbeh's life, but he did not elaborate.
The king met Wednesday with the pilot's family. His wife, Queen Rania, posted a message of support on Instagram: a Jordanian flag shaped like a fighter jet with the hashtag "We are all Moaz."
Kasasbeh, one of eight children, has been a pilot for six years, and was married in July. On Thursday, his father pleaded for his release, appealing to his captors as fellow Muslims.
"I direct a message to our generous brothers of the Islamic State in Syria: to host my son, the pilot Moaz, with generous hospitality," Safi Kasasbeh told reporters.
"I ask God that their hearts are gathered together with love, and that he is returned to his family, wife and mother," he said, adding, "We are all Muslims."
Although the Sunni militants claimed to have shot down the plane with an antiaircraft missile and posted photographs of Kasasbeh being led away, partially clothed and bleeding, they did not release any new information Thursday.
The pilot was part of the U.S.-led coalition airstrike campaign against Islamic State in Syria and Iraq, which began in September to retake territory the militants seized during a summer offensive.
The cause of the crash was still unclear Thursday.
The U.S. military said evidence "clearly indicates" that the militants did not shoot down the F-16.
"We can say with certainty that it was an aircraft crash and the plane was not downed by ISIL as was claimed by the terrorist organization," said Carl Hudson, a spokesman for U.S. Central Command, using an acronym for the group.
A Jordanian military official also told Petra state news agency Thursday that although the exact cause of the crash was still unknown, the plane was not downed by Islamic State fire.
But the pilot's relatives told reporters that the Jordanian government told them the plane was downed by a missile.
Younes Kasasbeh said the family was told that his nephew was flying at 400 feet on a bombing mission when his plane was struck by militants' heat-seeking missile and crashed in the Euphrates River.
Three other pilots wanted to rescue him but were afraid he might be killed because he had already fallen into enemy hands, and so they were ordered to return home, the uncle said.
Special correspondent Bulos reported from Amman and Times staff writer Hennessy-Fiske from Baghdad.