Trump's comments are the latest expressing the president's reluctance to impose penalties on Turkey for acquiring the S-400.
The US President Donald Trump said on Friday he does not blame Turkey for its acquisition of the Russian S-400 anti-air missile system.
"There's a lot of circumstances and a lot of problems that occurred during the Obama administration," Trump told reporters in the Oval Office. "This dates back to the Obama administration, which was a disaster."
The comments are the latest expressing the president's reluctance to impose penalties on Turkey for acquiring the S-400.
He did so last week when announcing Ankara's removal from the F-35 Joint Strike Fighter program. Turkey's suspension is slated to be completed by the end of March 2020.
Trump is further considering how to proceed with a law mandating sanctions for entities that conduct business with Russian defence firms.
The president signed the Countering America's Adversaries Through Sanctions Act into law in 2017. But he reportedly told Republican senators during a closed-door White House meeting on Tuesday that he is not in favour of implementing the penalties on Turkey.
Democrats were not invited to the meeting, but several Republicans who were present reportedly did not agree with the president's position.
Trump blames the Obama administration for the current row over its refusal to ink a deal with Turkey to sell it Raytheon's Patriot Missile systems.