* Iraqi Kurds say 17-km border trench is for security
* Rival Kurds over Syrian border call it divisive
By Isabel Coles
ARBIL, Iraq, April 17 (Reuters) - Iraqi Kurds are digging a 17-km (10-mile) trench on their border with Syria, reinforcing a political faultline between the two rival parties that dominate on either side of the frontier.
Iraqi Kurdish authorities say the ditch, which is approximately 3 metres deep and 2 metres wide, will help reduce smuggling and keep Islamist militants out of their relatively stable region as war grinds on in Syria.
But the Kurdish group that controls the Syrian side of the border says the ditch is designed to tighten a blockade against its enclave, and force it to submit to the authority of the ruling Kurdistan Democratic Party (KDP) in Iraq.
Whatever the motive, the ditch is highly symbolic, fortifying one of the frontiers regarded by many Kurds as a historic injustice that carved their ethnic homeland up into four parts spread across Iraq, Turkey, Iran and Syria.
It also illustrates the growing rifts and competition between Kurds across borders, and their ties to regional powers. People on the Syrian side protesting against the ditch have been shovelling soil and using their bare hands to refill it.
In Syria, the Democratic Union Party (PYD) has emerged as the most powerful Kurdish political group since civil war broke out more than three years ago, and in January declared self-rule in the country's northeast, bordering Turkey and Iraq.
Authorities in the Iraqi Kurdish province of Duhok abutting Syria denied the ditch was being dug for political reasons and said an official border crossing remained open for those who wished to travel back and forth.
"The measures that were recently taken in digging a ditch on the Syrian border are a result of the deterioration of security," read a statement posted on the Kurdistan Regional Government's (KRG) website.
Kurdistan has largely managed to insulate itself against the violence afflicting Syria and the rest of Iraq, providing refuge for thousands, but a bombing last year in the capital Arbil put the region on its guard.
That attack was claimed by the Islamic State in Iraq and the Levant (ISIL), which is active in Sunni Arab dominated provinces south of the region, and across the border in Syria, where it has been fighting Kurds.
"BARBED WIRE AND WALLS"
A senior KRG official said the ditch was no different from those dug around the cities of Arbil or Kirkuk for security, but the PYD has likened it to the "wall of shame" built by Turkey along its boundary with Syria's Kurdish areas.
Turkey is unnerved by the Kurds' growing clout in Syria, having fought for three decades on its own soil against the rebel Kurdistan Workers' Party (PKK), which is close to the PYD.
"At a time when everybody is looking forward for the elimination of barbed wire and walls dividing Kurdistan, digging border ditches ... is quite an eye-opening move," the PKK said in a statement.
"The ditch digging ... makes the enemies of the Kurds happy, for they have always benefited from the divisions and problems among the Kurds".
The PYD accuses those digging the trench of acting on the behest of Ankara, which has cultivated ties with the KDP, led by Iraqi Kurdistan's President Massoud Barzani.
The KDP backs several smaller parties in Syria that recently merged to form the Kurdistan Democratic Party of Syria (KDPS).
Even before work on the ditch began, the PYD complained it was not allowed to traverse the border freely, and last year took control of the Yaaroubiya crossing with Iraq, which lies beyond the control of Iraqi Kurdistan.
The PYD has since been using that border to get aid into Syria, with the approval of the Iraqi central government in Baghdad.
The controversy over the ditch is also being used by the KDP's rivals within Iraqi Kurdistan ahead of elections at the end of April.
A senior member of the Patriotic Union of Kurdistan (PUK) described the ditch as "unnecessary" and questioned why it had not been dug sooner if its real purpose was to secure the region.
"We are not happy about it," said a senior PUK member, on condition of anonymity. (Editing by Robin Pomeroy)