TOKYO (Reuters) -Oil prices fell on Friday but are set for a weekly gain, caught between expectations of lower demand in the U.S., the world's biggest oil user, as the summer demand period is ending and uncertainty around the availability of Russian supply.

Brent crude futures for October delivery, which will expire on Friday, fell 50 cents, or 0.7%, at $68.12 at 0126 GMT, while the more active contract for November slid 46 cents, or 0.7%, to $67.52.

West Texas Intermediate (WTI) crude futures were down 45 cents, or 0.7%, at $64.15.

Both benchmarks climbed on Thursday. Brent is set for a weekly gain of 0.6% while WTI is set to climb by 0.8%.

Prices gained earlier in the week after Ukrainian attacks on Russian oil export terminals but then dropped sharply as the market looked ahead to the end of the U.S. summer driving demand period with the Labor Day holiday on Monday and as more supply from major producers becomes available with the end of voluntary output cuts.

"Concerns that U.S. fuel demand will ease as the driving season ends after the Labour Day holiday weighed on the market," said Hiroyuki Kikukawa, chief strategist of Nissan Securities Investment, a unit of Nissan Securities.

"Still, uncertainty lingers over whether U.S. and Europe may tighten sanctions against Russia following its attack on Ukraine, and over the potential impact of U.S. tariffs on India, making investors reluctant to take large positions," he said.

Following the Ukrainian attacks on Sunday on Russia's Ust-Luga terminal, its main export site on the Baltic Sea for its Urals crude, operations will be cut by about half in September to about 350,000 barrels per day, sources said on Thursday.

Russian attacks on the Ukrainian capital Kyiv early on Thursday that killed 23 people have raised concerns the U.S. may respond with tighter sanctions.

Investors are also watching for India's response to pressure from the U.S. to stop buying Russian oil, after Trump doubled tariffs on imports from India to as much as 50% on Wednesday.

Still, Russian oil exports to India are set to rise in September, traders said, defying the U.S. pressure.

Saudi Arabia, the world's biggest oil exporter, may cut October crude oil prices for Asian buyers amid ample supply and weaker demand, refining sources said.

Russian crude supplies to Hungary and Slovakia through the Druzhba pipeline have restarted after an outage caused by a Ukrainian attack in Russia last week, Hungarian oil company MOL and Slovakia's economy minister said on Thursday.