* Libya's NOC protests to OPEC about Qatar's help for rebels
* Qatar is marketing Libyan crude on behalf of rebels
* OPEC has track record of working together despite disputes
(Adds details, OPEC no comment)
LONDON, April 18 (Reuters) - Libya's National Oil Corporation has protested to OPEC about help fellow member Qatar is giving to Libyan rebels, sources familiar with the matter said on Monday.
The complaint by Shokri Ghanem, the chairman of NOC, hints at rising political tension as a result of the Libyan crisis within the Organization of the Petroleum Exporting Countries, which pumps more than a third of the world's oil.
His letter to OPEC described the Qatari action as "very unfortunate," said one of the sources, who declined to be identified because the source is not authorised to speak to journalists.
Brent crude futures LCOc1 earlier this month rose above $127 a barrel, the highest in more than two and a half years, partly as a result of the conflict in Libya and resulting loss of most of its crude oil exports.
Qatar is marketing Libyan crude oil and buying fuel on behalf of the rebels -- throwing a lifeline to the forces fighting Muammar Gaddafi.
A second source familiar with the matter said NOC had complained to OPEC about Qatar, asking not to be named because of the political sensitivity of the issue. No-one from OPEC's Vienna headquarters was available to comment on Monday. OPEC has a track record of working together to limit oil supplies when it considers it necessary to support prices, even during times of political tension or war between members, such as following Iraq's 1990 invasion of Kuwait.
Although Saudi Arabia unilaterally adjusted supplies, the 12-member OPEC has yet to take any formal action in response to the loss of Libyan oil supplies.
The group is not scheduled to meet to reassess output policy until June.
Libya's oil output has fallen to less than 300,000 barrels per day (bpd) from 1.6 million bpd before the crisis, or almost 2 percent of world supply.