Turkey and Russia are haggling over the price for Turkey's purchase of advanced long-range S-400 anti-air missiles, billed as F-35 killers by Moscow.
Russian President Vladimir Putin and Turkish President Recep Tayyip Erdogan agreed on the outlines of an S-400 deal in May 3 meetings in the Black Sea resort town of Sochi.
"Now we have intensified the talks on some technical details and prices," Turkish Defense Minister Fikri Isik said Wednesday.
"As soon as the price issue is settled, our heads -- the president and the prime minister -- will make a final decision," he said, according to Turkey's Hurriyet newspaper.
The S-400 deal is expected to be a main topic of talks at the White House next Tuesday between Erdogan and President Donald Trump.
The U.S. has generally been opposed to having NATO allies such as Turkey acquire systems that do not mesh with NATO's common defense systems.
Isik appeared to brush aside the complaint, saying that NATO has ignored Turkey's need to upgrade its air defenses.
"They don't agree to the transfer of technologies in the sphere of air defense systems to us" and "do not make price concessions," he said. Then NATO allies "criticize us when we look for alternatives and hold negotiations with other countries."
The mobile surface-to-air S-400, called the "Growler" by NATO, costs about $400 million for a unit consisting of eight launchers and 112 missiles. Russia claims that the S-400 is effective against stealth aircraft such as the F-35 Joint Strike Fighter.
Russia deployed S-400 batteries to the Khmeimim Air Base near Latakia in northwestern Syria after a Turkish F-16C shot down a Russian Sukhoi Su-24M in November 2015 near the Turkey-Syria border.
Erdogan has since worked to restore relations with Russia, and the two countries, along with Iran, are now attempting to broker a peace deal in Syria to end the civil war.
On paper, the S-400 system radars can track targets out to 370 miles and fire missiles traveling at 10,500 mph to hit enemy aircraft and missiles at ranges up to 250 miles.
Russia, which boasts that the S-400 can engage 36 targets simultaneously, put the S-400 system on display at the annual Victory Day Parade in Moscow on Tuesday.
Turkey would be the third foreign buyer of the S-400. Last year, China and India agreed to terms on buying S-400 defense systems.