Once U.S. President Donald Trump returns from his overseas trip, the White House plans to launch its most aggressive effort yet to push back against allegations involving Russia and his presidential campaign, tackling head-on a scandal that has threatened to consume his young presidency.

Trump's advisers are planning to establish a "war room" to combat mounting questions about communication between Russia and his presidential campaign before and after November's presidential election, while bringing new aides into the White House, according to administration officials and people close to Trump.

The strategic shake-up comes as Republicans in Washington increasingly have fretted that the probe, continued chaos in the West Wing and Trump's steady slide in opinion polls will derail the president's drive to reform health care, cut taxes and rebuild the nation's infrastructure.