* Army trying to flush militants out of Chaambi area
* Al Qaeda's north African wing has claimed past attacks
* Porous Tunisia-Algeria border complicates operation (Adds details on fighters, defence minister)
By Tarek Amara
TUNIS, July 17 (Reuters) - At least 14 Tunisian soldiers were killed when dozens of gunmen with rocket-propelled grenades attacked two checkpoints in the remote Chaambi mountains, one of the deadliest militant strikes on the north African country's armed forces.
Since April, thousands of Tunisian soldiers have been deployed the Chaambi region bordering Algeria in an operation to flush out al Qaeda-linked militants. Some have been in the area since fleeing French intervention in Mali last year.
As many as 60 Tunisian and Algerian militants ambushed the checkpoints on Wednesday night, killing the soldiers as they were breaking their daily fast for the Muslim holy month of Ramadan, officials said. More than 20 soldiers were wounded.
"It was two simultaneous terrorist attacks when they were breaking their fast. The bodies of nine of them were burned after they were hit with an RPG. Five more were shot." Colonel Major Souhail Chmangi, chief of army land forces, said.
"This is open warfare," he said.
Another soldier is missing after the attack, and authorities could not confirm if he had been kidnapped. But Defence Minister Ghazi Jribi said the fighters had fled to Algeria.
Tunisia has struggled with the rise of radical Islamist militants since the 2011 popular revolt ended the rule of autocrat Zine el-Abidine Ben Ali and began its fragile steps towards democracy.
Militants calling themselves Okba Ibn Nafaa Brigade claimed responsibility on a social media site they often use. That claim could not be verified, but Tunisia says the group is operating in Chaambi and is tied to al Qaeda's north Africa wing.
Al Qaeda in the Islamic Maghreb, the north Africa branch, has also claimed attacks in Tunisia in the past, but another militant group, Ansar al Sharia, listed as a terrorist organisation by Washington, is also active.
The mountain range is tough terrain, allowing small groups of fighters plenty of cover. Tunisian forces have conducted several raids there and bombarded caves after eight soldiers were captured and executed last year.
Algeria's military, experienced in battling its own Islamist insurgency, has been coordinating with Tunisia, especially with intelligence-sharing. But despite their large presence, Tunisian troops have been harried by improvised landmines and the porousness of the border complicates the tracking of militants, who use the area as a training ground.
One of the Arab world's most secular states, Tunisia has adopted a new constitution and allowed a caretaker government to take over until elections this year as a way to ease tensions between a leading Islamist party and secular opponents.
But hardline, ultra-conservative Islamists are still influential, and Tunisia is one of the main sources of fighters travelling from north Africa to fight with Islamist groups in Syria and Iraq.
Last year a Tunisian man who had travelled to fight in Syria returned to carry out a suicide attack on a beach resort near the capital, killing only himself, but shocking a country that relies heavily on foreign tourism for revenue.
Tunisian officials also worry about arms and fighters spilling over from neighbouring Libya, where the weak government is unable to impose order on brigades of former rebels and militias still fighting since the 2011 fall of Muammar Gaddafi. (Writing by Patrick Markey; Editing by Louise Ireland)