US President Donald Trump is open to authorising additional strikes on Syria if the use of chemical weapons continues in the country, the White House says.

"The sight of people being gassed and blown away by barrel bombs ensures that if we see this kind of action again, we hold open the possibility of future action," White House spokesman Sean Spicer said at a briefing on Monday.

Earlier on Monday a US military spokesman said the United States military is still able to deconflict operations with Russia over Syria, but it was unclear how that was happening.

On Friday, Interfax news agency, citing a Russian Defense Ministry spokesman, said Russia had notified the Pentagon it would close down a communication line used to avoid accidental clashes in Syria.