Russia said on Monday it would be forced to respond in kind to restore the military balance with the United States if President Donald Trump carried through on a threat to quit a nuclear arms treaty and began developing new missiles.

Germany will seek NATO's help to maintain a nuclear arms treaty between Russia and the United States, and is ready to take action to force Moscow to comply with the pact, Foreign Minister Heiko Maas said on Tuesday.

U.S. national security adviser John Bolton said on Monday that Russian meddling in U.S. elections did not have any effect on the outcome, but told Russian officials that it did "sow enormous distrust of Russia" in the United States.

Under an unusual arrangement, Paul Manafort's attorney has kept Donald Trump informed about the former campaign chairman's meetings with prosecutors investigating Russian meddling in the 2016 U.S. election and, according to Trump's lawyer, Manafort has not said anything damaging about the president.

German Chancellor Angela Merkel on Monday called the killing of journalist Jamal Khashoggi at the Saudi consulate in Istanbul a "monstrosity" and vowed to halt all German arms exports to Riyadh until the case is cleared up.

Saudi Crown Prince Mohammed bin Salman met U.S. Treasury Secretary Steven Mnuchin in Riyadh on Monday, Saudi state TV said in a tweet.

British foreign minister Jeremy Hunt said on Monday it was not credible for Saudi authorities to claim that journalist Jamal Khashoggi died in a fight.

The United States wants to know all the details of how Saudi journalist Jamal Khashoggi was killed and talks on that are continuing with Saudi authorities, U.S. National Security Advisor John Bolton was quoted as saying on Monday.

President Donald Trump warned on Monday that the United States intended to build up its arsenal of nuclear weapons to pressure Russia and China.

President Donald Trump said Washington would withdraw from a landmark Cold War-era treaty that eliminated nuclear missiles from Europe because Russia was violating the pact, triggering a warning of retaliatory measures from Moscow.