PANAMA CITY, Feb 28 (Reuters) - Panama said on Thursday it was unsure of the authenticity of a purported threat by Colombia's FARC rebels to kidnap Panamanian politicians and police in response to the capture of six FARC combatants.

"We are not sure of its authenticity, and so we are not going to make any comment about its content or anything else," Interior Minister Daniel Delgado said of a statement that appeared to be signed by a FARC guerrilla commander threatening retaliation for the arrests.

"We are investigating but we are not giving it credit," Delgado told Reuters.

The FARC -- Revolutionary Armed Forces of Colombia -- is fighting Latin America's oldest running rebel insurgency, although it has been weakened and driven back by President Alvaro Uribe's U.S.-backed security campaign.

Delgado said it was not clear where the document originated and noted it was not posted on a Web site where FARC statements usually appear.

In what appeared to be a rare threat against neighboring Panama, a FARC commander identified as Bercerro in a statement sent to Panamanian media on Wednesday said Panama had until March 1 to free the rebels or face abductions.

The six fighters were captured at sea during the weekend near the remote Darien jungle area between Colombia and Panama.

"If they do not comply with this, we have instructions to take necessary hostages from among members of the national police, officials and local politicians to force an exchange," the statement said.

The statement appeared in the same format and style as previous FARC communiques. The FARC leadership often use two Web sites to release statements, but communiques from regional rebel fronts are issued separately.

The Darien area has long been a difficult region, where illegal armed groups and smugglers are common.

Violence from Colombia's four-decade-old conflict is down, but the rebels still are holding dozens of hostages they want to swap for jailed rebels. Among high-profile captives are French-Colombian politician Ingrid Betancourt and three Americans.

(Reporting by Elida Moreno, writing by Catherine Bremer, editing by Vicki Allen)

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