President Trump likely sent a pair of tweets about the special counsel investigation on Thursday in response to reports about special counsel Robert Mueller's newest line of inquiry, White House deputy press secretary Sarah Sanders said.

"There were some developments yesterday," Sanders said of why the president took to Twitter on Thursday to criticize the special counsel.

Trump, who has stayed relatively quiet about the investigation into allegations of Russian collusion since Mueller was named special counsel, slammed reports that the probe had expanded to include obstruction of justice accusations.

"They made up a phony collusion with the Russians story, found zero proof, so now they go for obstruction of justice on the phony story. Nice," Trump tweeted Thursday morning before describing the probe as a "witch hunt."

Sanders said earlier this week that Trump had no plans to remove Mueller, despite criticism of the special counsel from people close to him.

"I believe so," Sanders said when asked whether the president remains confident in Mueller.

The White House has declined to answer most questions about the Russian investigation, instead referring reporters to Trump's outside counsel, Marc Kasowitz.

"There have been very clear guidelines set that we have relayed to you guys multiple times over," Sanders said Thursday after facing several questions about Trump's attitude toward Mueller.