Spanish intelligence agents were working urgently on Wednesday to track down three journalists from Spain, the latest foreigners to go missing while reporting in war-torn Syria, AFP reported.
Prime Minister Mariano Rajoy said "no theory is being ruled out" in relation to the disappearance of the three, missing for 10 days in a war zone where numerous foreigners have been kidnapped in the past.
Spain's foreign minister said embassies and the intelligence service were "fully active" in the search.
The journalists have not been heard from for 10 days. They had been reporting in Aleppo, a city in northwestern Syria that has been devastated by fighting, the Spanish press federation FAPE said.
Officials could not yet confirm whether the men had been kidnapped.
The case raised memories of three other Spanish reporters who were seized by the jihadist group Islamic State (ISIS) in northern Syria in 2013 and released in March 2014.
Foreign Minister Jose Manuel Garcia-Margallo called for caution over reports that the Spaniards were seen being taken away by men in an area of Aleppo controlled by several rebel groups.
"No demands have been made so far" by any would-be captors, said Garcia-Margallo.
"Leave us to work discreetly, because believe me, that is the best thing for your colleagues," he said, adding "there are people from (Spain's) National Intelligence Centre in Syria helping us".
A local monitoring group, the Syrian Observatory for Human Rights, cited witnesses saying the three were seen being taken away in the Maadi district on July 13.
Garcia-Margallo said he had been in contact with various embassies, United Nations representatives as well as Spanish intelligence operatives in Syria.
"No theory is being ruled out. The only thing we know for sure is that they have disappeared," Rajoy said in comments quoted by Spanish news agencies and confirmed by an official source.
Media rights group Reporters Without Borders ranks Syria as the most dangerous country in the world for journalists.
It says at least 44 journalists have been killed since the conflict broke out in 2011 in Syria, where various armed factions are battling President Bashar Al-Assad's regime and each other.
In August 2014, Islamic State decapitated American journalist James Foley, who was seized in northern Syria in 2012. It followed that by beheading several other hostages, including journalist Steven Sotloff, as well as aid workers Alan Henning and Peter Kassig.
In 2013 three other Spanish journalists were seized by ISIS: El Mundo correspondent Javier Espinosa, freelance photographer Ricardo Garcia Vilanova and Marc Marginedas of El Periodico newspaper. They were all released.
AFP contributed to this report.