By Lidia Kelly

WARSAW (Reuters) - Poland and the United States are set to agree the sale of Raytheon Co's Patriot missile defence system to Warsaw next week, three sources told Reuters.

Poland has previously said it was planning to spend around $7.6 billion on the project, but the sources said the deal to be announced next week may not be more than $5.5 billion.

NATO member Poland has sped up efforts to overhaul its military following Russia's annexation of Ukraine's Crimea peninsula in 2014 and in response to Moscow's renewed military and political assertiveness in the region.

The U.S. Department of State approved the sale of the systems in November, with Poland interested in buying eight of the systems.

"The letter of agreement is to be signed on March 28," a source close to the defence ministry said. Two others sources familiar with the matter confirmed the plan.

Patriot missile defence interceptors are designed to detect, track and engage unmanned aerial vehicles (UAVs), cruise missiles and short-range or tactical ballistic missiles.

Poland's defence ministry did not respond to Reuters' request for comment, but Defence Minister Mariusz Blaszczak said in late January that he expected the deal to be signed by the end of the first quarter.

The proposed sale includes 208 Patriot Advanced Capabilty-3 (PAC-3) Missile Segment Enhancement missiles, 16 M903 launching stations, four AN/MPQ-65 radars, four control stations, spares, software and associated equipment.

In addition, Poland is authorized to buy U.S. government and contractor technical, engineering and logistics support services as well as range and test programs.

The deal approved in November by the U.S. State Department envisaged the sale totaling up to $10.5 billion, but two sources told Reuters that Poland has received different options with different individual components and different offset scenarios. The sources estimated the options at up $4.5 billion - $5.5 billion, but said the negotiations are not over yet.

(Writing by Lidia Kelly; Editing by Susan Fenton)