Washington, Oct 19 (IANS): US Deputy Attorney General Rod Rosenstein has not given special counsel Robert Mueller a date by which his investigation into Russian interference in the 2016 election must be completed, a Justice Department source said.

But speculation that it will wrap up immediately after the November midterm elections is off the mark and instead, the probe is expected to wind down by the end of the year, the source told CNN on Thursday.

"Mueller and his team have to provide a confidential report to Rosenstein at the end of the investigation," the source said.

"As far as timing, the investigation is obviously ongoing, and I'd expect it to continue well after the midterms."

Rosenstein, who oversees the investigation, on Wednesday called the probe "appropriate and independent", in an interview with The Wall Street Journal, but declined to comment on when it would end.

How and when exactly Mueller's investigation will finally conclude has captured significant public attention as President Donald Trump fiercely maintains that his campaign did not collude with the Kremlin.

To date, the investigation has yielded guilty pleas from four former campaign officials, but Trump has complained that the investigation was unfounded and a waste of taxpayer money.