ATHENS, Greece: Greece has agreed to spend up to US$4 billion on U.S. weapons and upgrades to its fighter jet fleet as the NATO member pushes ahead with a broader modernization of its armed forces, two senior sources said.

The agreement covers advanced missiles and bombs as well as upgrades to more of Greece's F-16 fighter jets, and comes a day after U.S. Secretary of State Marco Rubio visited Athens.

Greece had already planned to spend about 28 billion euros by 2036 to buy U.S.-made F-35 fighter jets and helicopters, and to build the "Achilles Shield," a multi-layer anti-ballistic and anti-aircraft system with Israeli-made weapons.

"The agreement includes new advanced missiles and bombs for the F-16 Viper, the F-35 fighter jets, and the Black Hawk helicopters. Also, the upgrade of old F-16 fighter jets to the Viper version," a senior defense Ministry official told Reuters.

The official added that most of the procurements have been included in the Government's multi-year defense plan and will not put additional pressure on the country's finances.

A senior foreign ministry official confirmed the details, adding that the cost will be spread over several years.

Rubio said in Athens on October 7 that the two NATO allies had agreed on a joint security plan to share counterterrorism information between their law enforcement agencies.

Greece agreed in 2024 to buy 20 Lockheed Martin F-35 fighter jets and 35 Sikorsky UH-60M Black Hawk helicopters. It is upgrading about 82 F-16s to the Viper version and wants to upgrade 37 more Block 50s to Vipers, costing about $1 billion.

The move is part of Greece's bid to modernize its armed forces as it emerges from the 2009-18 debt crisis that led to years of austerity.

Greece spends nearly 3.5 percent of its gross domestic product on defense, a higher proportion than many other NATO countries, due to its long-standing dispute with Turkey.