UPDATE 5-Brent buoyed by Iran threat, U.S. GDP weighs

* Europe worries keep risk appetite in check

* Iran parliament to debate halting exports

* U.S. GDP data disappoints (Updates with GDP data, updates prices, adds quote)

By Simon Falush

LONDON, Jan 27 (Reuters) - Brent crude held above $111 on Friday as comments from Iran that it might stop exports to the European Union intensified worries about security of supply, but weaker than forecast economic data from the United States capped gains.

Brent crude rose 85 cents to $111.64 a barrel by 1445 GMT. It was up around 1.4 percent so far this week, after two weeks of losses. U.S. crude fell 59 cents to $100.29.

A law to be debated in Iran's parliament on Sunday may halt oil exports to the European Union as early as next week, foiling an EU plan to phase in an oil embargo gradually to help its struggling economies adapt, lawmakers said on Friday.

If Iran stops crude exports to the EU earlier than July when an EU ban on Iranian crude takes effect, it could cause problems for some countries which are heavily dependent on the oil.

Royal Dutch Shell will implement the terms of a European Union embargo on Iranian crude but will need some time to study details of the sanctions which are likely to push oil prices higher, Chief Executive Peter Voser said on Friday.

The premium from geopolitical risk was offset by data that was not as positive as the market had anticipated.

The U.S. economy grew at its fastest pace in 1-1/2 years in the fourth quarter at 2.8 percent, but this was under the 3.0 percent that had been forecast, prompting investors to trim bets in demand sensitive assets.

Oil came off the day's peak, but was relatively resilient.

"Oil's come off with the data, but it's not collapsing because Iran is making noises that it might vote to cancel contracts with Europe," said Rob Montefusco, broker at Sucden Financial.

EUROPE WORRIES

Along with the disappointing U.S. data, worries about prospects for the euro zone due to ongoing uncertainty about the debt situation also kept oil's gains in check.

Portuguese five and 10-year government bond yields were set to remain under pressure after hitting euro-era highs on Thursday as fears grew that the country may follow Greece in requiring another bailout or seeking to restructure its debt.

Athens is locked in tough negotiations with its private creditors on a restructuring it needs quickly to avert a disorderly default when a major bond redemption falls due in March.

Greece's bondholders are demanding the European Central Bank contributes to a deal to put the country's messy finances back on track.

(Additional reporting by Manash Goswami in Singapore; editing by James Jukwey)