Worries are growing about the whereabouts of a freelance Japanese journalist, last heard from one month ago in war-torn Syria, where reporting assignments have become among the most precarious in the world.

It is not known why Jumpei Yasuda, who has been reporting on the Middle East since 2002, has not been in contact. He may not have access to communications or if he has been taken captive.

Yasuda was taken hostage in Iraq in 2004, with three other Japanese, but was freed after Islamic clerics negotiated his release.

Kosuke Tsuneoka, another freelance reporter, said Wednesday that he received a message from Yasuda in Syria on June 23, but has not heard from him since.

"It is not normal that there has been no contact from him at all," Tsuneoka said in a telephone interview, adding that no one should jump to conclusions about Yasuda's fate.

The Japanese Foreign Ministry said it was aware of the reports but has no confirmed information on Yasuda.

Three Spanish freelance journalists, who entered Syria separately from Yasuda, Antonio Pampliega, Jose Manuel Lopez and Angel Sastre, were also reported missing this week by a Spanish journalism association.

The four-year conflict in Syria has killed more than 220,000 people, and has been the scene of killings and kidnappings by Islamic State militants, including that earlier this year of freelance Japanese journalist Kenji Goto and Haruna Yukawa, who ran his own security company.

Most media organizations, including mainstream Japanese ones, have pulled out of Syria. The Japanese government warns against travel to Syria. It is not known how many foreign and local journalists remain held in Syria, although the number is likely in the dozens.

Yasuda, who reported from Afghanistan and Iraq, wrote a book five years ago about laborers in war zones. He worked as a cook in Iraq for nearly a year to research his book.

His last Tweet was sent June 21, when he complained that his reporting activities were often obstructed and that he would stop Tweeting his whereabouts and his activities. He did not give details.

"What you see by going to places is limited, especially in war, and no one goes there, thinking everything can be understood by just being there," Yasuda tweeted June 18.