UN Secretary General Antonio Guterres expresses concern over situation spiralling out of control.
Any US military strikes against Syria are likely to come from the navy, given the threat to aircraft posed by Russian and Syrian defence missiles.
Donald Trump has also been liaising with Turkish President Recep Tayyip Erdoğan. The United States has military bases in Turkey and the leaders of the two countries say they have agreed to stay in close touch.
But White House spokeswoman Sarah Sanders said Trump was also reviewing non-military options, and that he's still assessing how to respond.
"The President has not laid out a timetable," she said. "And he's still leaving a number of other options on the table, and we are still considering a number of these and a final decision on that front hasn't been made. The President holds Syria and Russia responsible for this chemical weapons attack."
Russian President Vladimir Putin on Wednesday said the situation could "only give rise to concern".
Speaking at the Kremlin, he expressed hope that "common sense" would prevail and that international relations would become "more stable and predictable".
The United Nations security council is meeting again today to discuss the situation.
UN Secretary-General Antonio Guterres has expressed his "deep concern about the risks of the current impasse", stressing the need to avoid the situation spiralling out of control."