By Jim Wolf

WASHINGTON (Reuters) - The Obama administration has told U.S. lawmakers it is delaying a planned $53 million arms sale to Bahrain, a key Gulf ally, pending the outcome of a local investigation into alleged human-rights abuses since an uprising in February.

The handling of the issue is sensitive because of U.S. security interests in Bahrain, host of U.S. naval headquarters in the Gulf for more than 60 years and a pivot for U.S. efforts to deter Iran.

The State Department, in an October 14 reply to members of Congress, said it would hold off until it could review the findings of a Bahrain "Independent Commission of Inquiry" due to report to the nation's king on October 30.

The department said it expected the report to be made public shortly. It would then assess the government's efforts to implement the recommendations and make "needed reforms."

King Hamad bin Isa Al Khalifa convened the five-member commission in June to investigate the violent crackdown on the mainly Shi'ite protesters who rose up after the revolt that overthrew Egyptian President Hosni Mubarak on February 11.

"We will weigh these factors and confer with (the U.S.) Congress before proceeding with additional steps related" to the proposed arms sale notified to Congress on September 14, David Adams, assistant secretary of state for legislative affairs, wrote to Senator Ron Wyden and others who had questioned the deal and its timing.

The sale has become unusually controversial in Congress because, critics say, it reflects a double standard compared to the U.S. response to the 2011 unrest in Egypt, Tunisia, Libya, Syria and Yemen.

Wyden and Representative James McGovern, both Democrats, have introduced resolutions to prevent the sale "until meaningful steps are taken to improve human rights" there.

Senator Bob Casey, among the lawmakers who had raised such concerns with the State Department, welcomed the delay.

Completing an arms sale to Bahrain under the current circumstances "would weaken U.S. credibility at a critical time of democratic transition in the Middle East," he said in a statement to Reuters.

PRESSURE FROM SAUDI ARABIA

The administration is under pressure to stick by Bahrain's ruling Sunni Muslim family, notably from Sunni-led Saudi Arabia and other members of the Gulf Cooperation Council.

The Sunni ruling family put down the pro-democracy uprising with the help of neighboring Saudi Arabia and other Gulf Cooperation Council members, spearheaded by a reported 1,200 Saudi forces in 20 tanks and in other armored vehicles.

A key U.S. concern is that a fall of the Al Khalifa family and rise of a Shiite-led government could boost Iran's influence and lead to a loss of access to Bahrain's military facilities and U.S. influence in the region.

Kenneth Katzman, an expert on the region at the nonpartisan Congressional Research Service, said Saudi Arabia and its close Gulf allies had made clear they would not let the Bahraini government fall, only to be replaced by majority Shi'ites.

The sale, if it goes through, "would demonstrate the administration's intent to continue to engage the Bahrain government and not penalize it over alleged human-rights abuses against the Shi'ite opposition," Katzman said.

At stake is the proposed U.S. supply of 44 "Humvee" armored vehicles and several hundred TOW missiles along with associated equipment and support, worth an estimated $53 million.

Prime contractors for the deal would be privately held AM General and Raytheon Co, the Pentagon's Defense Security Cooperation Agency, which administers foreign arms sales, told Congress last month.

Bahrain is the headquarters of the U.S. Fifth Fleet. The United States has designated Bahrain as a "major non-NATO ally," paving the way for what are supposed to be expedited arms deliveries.

About 30 people, mainly Shi'ites, died after the protest movement erupted in Bahrain in February.

(Reporting by Jim Wolf in Washington; editing by Todd Eastham)