* Move comes as President Erdogan launches rights' action plan
* Pro-Kurdish MPs already targeted in many court cases
* Turkey has long history of banning political parties
By Orhan Coskun and Daren Butler
ANKARA, March 3 (Reuters) - Turkey's top appeals court has launched an enquiry into the pro-Kurdish Peoples' Democratic Party (HDP) over alleged links to militants in a step that could ultimately lead to a ban on the third biggest party in parliament, officials said on Wednesday.
The move coincided with President Tayyip Erdogan's pledge on Tuesday to strengthen freedom of expression and rights to a fair trial in an "action plan" that critics said did not address concern about an erosion of human rights in Turkey.
The HDP has faced growing pressure after Ankara said last month Kurdistan Workers Party (PKK) militants executed 13 prisoners, including Turkish military and police personnel, during an army operation to rescue them in Iraq's Gara region.
A senior member of Erdogan's ruling AK Party on Tuesday endorsed nationalist calls for the closure of the HDP, which has 56 members in the 660-seat assembly. It is accused of links to the banned PKK, which it denies.
"The judiciary has begun an enquiry," an AKP official told Reuters. "The appeals court has sprung into action. The possibility of it being closed appears pretty high."
Turkey has a long history of banning parties although they have often been able to re-form under new names. The official said measures may be taken to prevent this happening again.
A crackdown on the HDP in recent years has included the arrests of thousands of party officials and members, while dozens of its elected mayors and lawmakers have been ousted. It was defiant over the latest pressure.
"The efforts to shut us down will diminish them further and make us grow more," HDP co-leader Mithat Sancar told his party's lawmakers on Tuesday, recalling the banning of a previous pro-Kurdish party in 1994.
"After they closed the party the same political tradition grew in waves," he said. "They will see very clearly that we will emerge stronger from this tyrannical darkness."
Many HDP deputies already face bids to lift their immunity from prosecution. The party's jailed former leader is among those charged over deadly 2014 protests in Turkey calling for action to protect Kurds in Syria from Islamic State.
Another official said the appeals court had sought from prosecutors copies of the indictment over the 2014 protests and details of cases against HDP deputies, and will examine whether the party is a focus for PKK activities.
A nationalist party allied to Erdogan's government repeated on Tuesday its call for the HDP's closure over links to the PKK, which Turkey, the European Union and United States designate a terrorist group.
More than 40,000 people have been killed in the conflict since the PKK launched its insurgency in mainly Kurdish southeast Turkey in 1984. (Writing by Daren Butler Editing by Peter Graff)
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