The Indian government is attempting to buy five Russian-built S-400 missile defense systems in a $4.5 billion deal, sending a team of negotiators to Moscow to finalize it. India’s Defense Acquisition Council, the highest procurement body under the Ministry of Defense, cleared the purchase of the advanced missile system during a meeting Thursday.

Moscow, however, is said to be lukewarm on the deal, preferring to concentrate on completing a joint venture to build the T-50 stealth fighter jet, the first fifth-generation fighter to be built outside of the U.S., according to a Defense News report released late Thursday. While a prototype of the advanced fighter already exists, Moscow wants India to commit to building and using the T-50, a deal which will release $10 billion to Moscow. The T-50 is due to be put into mass production in late 2016 regardless is India walks away from the deal. 

Even if Moscow can be convinced to sign the S-400 deal, it’s unlikely to be completed before Indian Prime Minister Narendra Modi meets Russian President Vladimir Putin on a state visit to Moscow over Dec. 24 – 25, according to the Defense News report.

India wants the Russian missile system to fill a gap in its own defense while it continues to develop its own system, which may take two to three years. As things stand, India has no protection against ballistic missiles or stealth aircraft, which the S-400 is supposed to be able to beat.

The S-400 is capable of intercepting and destroying airborne targets at distances up to 400 kilometers (250 miles), while engaging six targets at once. The domestic version that India is still building will not be as good as the Russian system, which is said to be more advanced than the U.S.-built Patriot missile system, according to Russian reports. 

India is one of Russia’s biggest defense customers, spending billions each year to obtain relatively cheap and technologically advanced weapons from Moscow.