WASHINGTON, July 29 (Reuters) - U.S. lawmakers have put on hold a proposal to sell almost $1 billion of weapons to Nigeria over concerns about possible human rights abuses by the government, three sources familiar with the matter said on Thursday. The proposed sale of 12 AH-1 Cobra attack helicopters made by Bell (TXT.N) and related equipment worth $875 million is being delayed in the Senate Foreign Relations Committee and in the House Foreign Affairs Committee, the sources told Reuters on condition of anonymity. Foreign Policy reported earlier this week that the State Department had informally notified Congress of the proposed sale but it was frozen in the Senate committee. The package includes a dozen helicopters, spare engines, navigation systems, and 2,000 precision guided munitions, it said. Under normal practice, the State Department tells Congress of proposed arms sales informally in advance to give lawmakers the chance to put a hold on the proposals to raise concerns. If Congress opposes the sale after a formal notification, it can pass legislation to block it. A State Department spokesperson said: "As matter of policy, we will not confirm or comment on proposed defense sales until they have been formally notified to Congress." The Senate and House Committees both declined to comment on the issue. A spokesman for Nigerian President Muhammadu Buhari also declined to comment. The sale that has been put on hold could have an impact on Nigeria"s efforts to seek support to fight Islamic State-allied group Boko Haram in the northeast and armed bandits in the northwest of the country. Nigeria is also battling rising armed robberies and kidnappings for ransom where thinly deployed security forces have struggled to contain the influence of armed gangs.[nL8N2OX28A] U.S. officials last October complained of "excessive force" by Nigerian military forces on unarmed civilians and called for restraint after soldiers opened fire on protesters demonstrating against police brutality in Lagos. Thousands of Nigerians protested nationwide for nearly two weeks last October, demanding an end to a police unit called the Special Anti-Robbery Squad (SARS), which they said was responsible for extortion and human rights abuses. The police disbanded SARS, but denied most accusations.