By Ece Toksabay

ANKARA (Reuters) - Turkey's government is blocking access to independent investigations into alleged mass abuses against civilians in southeast Turkey, where security forces are fighting Kurdish militants, Human Rights Watch said on Monday.

Thousands of civilians have been caught up in fighting between the Kurdistan Workers Party (PKK) militants and security forces that flared in July after a two-year-old ceasefire collapsed.

U.S.-based Human Rights Watch said in a report that alleged abuses in the conflict included unlawful killings of civilians, mass forced civilian displacement, and widespread unlawful destruction of private property. It said at least 338 civilians had died after being caught up in clashes.

In a release on its website, Human Rights Watch called on the government to promptly grant the United Nations Office of the High Commissioner for Human Rights permission to enter the area and investigate in accordance with its standards.

The Turkish government denies targeting civilians in the operations. A presidency official described the HRW report as inaccurate and misleading.

Turkey, the United States and the European Union all classify the PKK as a terrorist organization. The PKK says it is fighting for autonomy for Turkey's Kurdish minority.

According to the Turkish army, more than 600 militants were killed in security operations in Cizre, a town of more than 100,000 people that was sealed off and under curfew for 78 days until March 2. It denies accusations it has killed large numbers of civilians.

"The Turkish government's effective blockade of areas of the southeast fuels concerns of a major cover-up," said Emma Sinclair-Webb, senior Turkey researcher at Human Rights Watch.

"The Turkish government should give the UN and non governmental groups immediate access to the area to document what's going on there."

More than 40,000 people have been killed in the conflict since the PKK began its insurgency in 1984.

(Additional reporting by Gulsen Solaker, editing by Dasha Afanasieva and Ralph Boulton)