By Humeyra Pamuk
ISTANBUL (Reuters) - Turkey's military outposts on the border with Syria came under fire from Kurdish militia across the frontier, the army said on Wednesday, a day after Turkish warplanes bombed groups in northeastern Syria and Iraq affiliated to the outlawed Kurdistan Workers Party (PKK).
Four outposts of the Turkish army along the Syria border were attacked on Wednesday with artillery and mortar shells, the army said.
Three of the attacks were carried out from territory held by the U.S.-backed Kurdish YPG militia, while the other originated from a region under the control of the Syrian government. There were no casualties but minor material damage.
YPG spokesman Redur Xelil said there was heavy Turkish artillery fire across the border into Syria on Wednesday. The shelling hit near Darbasiya, a town in a Kurdish-controlled part of northeast Syria.
The Syrian Observatory for Human Rights war monitor reported clashes between Kurdish fighters and Turkish forces along the border near Darbasiya.
Tensions in the area escalated after Turkish warplanes struck Kurdish targets in Iraq's Sinjar region and northeast Syria, killing about 70 inside the two neighboring states, according to a Turkish military statement.
The air strikes in Syria targeted the YPG, a key component of the Syrian Democratic Forces (SDF), which are backed by the United States and have been closing in on the Islamic State bastion of Raqqa.
The Turkish raids showed the challenges facing U.S.-led attempts to defeat Islamic State in Syria and tension between NATO allies Washington and Ankara over Kurdish combatants who have been crucial in driving back the jihadists.
Ankara sees the YPG as a hostile force and an extension of the outlawed PKK, which has waged a three-decade insurgency against the Turkish state and is considered a terrorist group by Turkey, the United States and the European Union.
On Wednesday, Turkish warplanes continued cross-border raids, striking Kurdish militant targets in northern Iraq and killed six militants, the army said.
Turkey's strikes drew criticism from the United States and Russia. Turkish President Tayyip Erdogan, who has long said Ankara would not allow Iraq's Sinjar to become a PKK base, said all partners were informed ahead of the operation.
The Turkish military gave the U.S.-led coalition against Islamic State less than an hour of advance notice, a U.S. coalition spokesman said on Wednesday, describing it as inadequate time to ensure the safety of forces on the ground.
"That's not enough time and this was...not coordination as you would expect from a partner and an ally in the fight against ISIS," U.S. Air Force Colonel John Dorrian told a Pentagon teleconference.
Turkish foreign ministry spokesman Huseyin Muftuoglu said the partners were informed through both military and diplomatic channels.
The Combined Air Operations Center in Qatar, responsible for providing command and air control in regions including Iraq and Syria, was also informed in advance, Muftuoglu said.
The Russian foreign ministry said Turkey's had violated the fundamental principles of intergovernmental relations.
(Additional reporting by Ece Toksabay and Tuvan Gumrukcu in Ankara, Lisa Barrington in Beirut and Rodi Said in Qamishli, David Alexander in Washington; Editing by Ralph Boulton)