ADEN, Yemen (Reuters) - Yemeni tribesmen released three kidnapped employees of the International Committee of the Red Cross, the humanitarian agency and a Yemeni security source said on Thursday.

The men - a Swiss, a Kenyan and a Yemeni national - were kidnapped on Monday in the southern province of Abyan. They had been freed late on Wednesday and were in good health, the source said.

The ICRC, which did not disclose the workers' nationalities, said they were safely back in Aden. They had been stopped in an ICRC-marked vehicle on their way back from a field trip by armed men, the ICRC said.

Kidnappings of Westerners occur sporadically in Yemen, mostly carried out by al Qaeda militants or tribesmen.

Lawlessness in the poor Arabian Peninsula state has alarmed neighboring Saudi Arabia, the world's leading oil exporter, as well as the United States, which increasingly views Yemen as a front line is its struggle against al Qaeda.

(Reporting by Mohamed Mushashaf; Writing by Amena Bakr; Editing by Yara Bayoumy, John Stonestreet)