SINGAPORE (Reuters) -Brent oil prices fell on Tuesday as sluggish economic growth in China, the world's biggest crude importer, increased worries about demand that overshadowed the impact of the halt of production and exports from Libya.

Brent crude futures were down 37 cents, or 0.5%, to $77.15 a barrel by 0525 GMT.

West Texas Intermediate crude futures, which did not settle on Monday because of the U.S. Labour Day holiday, were up 29 cents, or 0.4%, at $73.84 a barrel.

"Oil remains under pressure given lingering Chinese demand concerns. Weaker than expected PMI data over the weekend would have done little to ease these worries," said Warren Patterson of ING, adding that demand jitters are offsetting the Libyan supply disruptions.

China's purchasing managers' index (PMI) hit a six-month low in August. On Monday, the country reported new export orders in July fell for first time in eight months, and new home prices grew in August at their weakest pace this year.

In Libya, oil exports at major ports were halted on Monday and production curtailed across the country, six engineers told Reuters, continuing a standoff between rival political factions over control of the central bank and oil revenue.

The country's National Oil Corp (NOC) declared force majeure on its El Feel oil field from Sept. 2. Total production had plunged to little more than 591,000 barrels per day (bpd) as of Aug. 28 from nearly 959,000 bpd on Aug. 26, NOC said. Production was at about 1.28 million bpd on July 20, the company said.

Still, some supply is set to return to the market as eight members of the Organization of the Petroleum Exporting Countries (OPEC) and affiliates, known as OPEC+, are scheduled to boost output by 180,000 bpd in October. The plan is likely to go ahead regardless of demand worries, according to industry sources.

OPEC planners may decide that the expected upcoming cuts in U.S. interest rates and the Libyan outage provides space for the addition of more oil, said RBC Capital analyst Helima Croft in a note.

"In our view, a prolonged Libyan outage could support Brent prices" around $85 a barrel, even with additional supply coming onto the market in the fourth quarter, she said.

A Reuters survey on Monday found OPEC's oil output last month fell to its lowest level since January.

Continuing disruptions to supply flows from the Middle East are also supporting the market. Two oil tankers were attacked on Monday in the Red Sea off Yemen but did not sustain major damage. The Iran-backed Houthis, who are attacking shipping in support of Hamas' fight against Israel in Gaza, claimed responsibility.