Brent crude futures were down 37 cents, or 0.48%, to USD 77.15 a barrel by 0156 GMT.
U.S. West Texas Intermediate crude, which did not have a Monday settlement because of the U.S. Labour Day holiday, was 28 cents up from its Friday close of USD 73.55.
"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.
China's purchasing managers' index (PMI) hit a six-month low in August. On Monday, China posted the first decline in new export orders in eight months in July, and said new home prices grew in August at their weakest pace this year.
"These demand jitters are clearly more than offsetting the supply disruptions from Libya," Patterson said.
The United Nations Support Mission in Libya said it held talks on Monday to resolve a dispute over control of the central bank that triggered a blockade of the country's most valuable commodity, sending oil production to less than half of its usual level.
Rival factions concluded a draft agreement and aimed to sign it on Tuesday, the UN said without providing further details.
Oil exports at Libyan ports remained halted on Monday and production curtailed, six engineers told Reuters.
Libya's National Oil Corp (NOC) said on Monday it had declared force majeure on its El Feel oil field from Sept. 2.
Total production had plunged to little more than 591,000 bpd as of Aug. 28 from nearly 959,000 bpd on Aug. 26, NOC said. Production was at about 1.28 million barrels per day (bpd) on July 20.
Eight members of the Organization of the Petroleum Exporting Countries and affiliates, known as OPEC+, are scheduled to boost output by 180,000 bpd in October, a plan industry sources said is likely to go ahead regardless of demand worries.
"There are suggestions they will stick to their planned increase, however much will depend on how much more weakness we see in the market," said ING's Patterson.
A Reuters survey on Monday found global oil output last month fell to its lowest level since January.
Exacerbating supply concern, two oil tankers were attacked on Monday in the Red Sea off Yemen but did not sustain major damage. The Iran-backed Houthis claimed responsibility.
Also, Russia's Gazpromneft Moscow refinery suspended operations at one unit for repairs. A fire broke out on Sunday after a drone strike at the plant, which processed 11.6 million tons of crude oil last year.
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