SIDON, Lebanon, Aug 23 (Reuters) - Two members of the Palestinian group Fatah were killed in worsening clashes with Islamist groups in a Palestinian refugee camp in Lebanon on Wednesday, medical sources said, bringing the number of dead to six in nearly a week of fighting.

The clashes at the Ain el-Hilweh camp began late last Thursday between Islamist fighters and a joint force comprising the main Palestinian factions, including Fatah, which are responsible for the camp's security.

Two other Fatah members and two Islamist fighters had already been killed and a total of 15 combatants and civilians have been wounded, a security source said.

The source said clashes had escalated on Wednesday, when gunfire wounded three people, including two Lebanese security personnel, outside the camp.

The fighting was triggered when a leader of an armed faction sympathetic to the Islamist Badr group fired at the headquarters of the joint security force last week.

In April, seven people were killed in clashes in the camp between the Badr group and the joint security force after it deployed there.

Lebanon's Palestinian camps, which date back to the 1948 war between Israel and its Arab neighbours, mainly fall outside the jurisdiction of Lebanese security services. There are some 450,000 Palestinian refugees living in 12 camps in Lebanon.

(Writing by Sarah Dadouch; Editing by Angus McDowall and Mark Trevelyan) ((Sarah.Dadouch@thomsonreuters.com;))