london british prime minister theresa may speak monday u.s. president donald trump agree window opportunity exist persuade russia break tie syrian president bashar al-assad may office say spokeswoman prime minister say trump thank may support follow last week u.s. military action syria assad regime white house late monday say trump speak may separately german chancellor angela merkel telephone u.s. attack thank support say statement may merkel express support u.s. action agree trump importance hold assad accountable shift washington strategy u.s. missiles hit syrian air base last week retaliation unite state ally say poison gas attack syria military score civilians die syrian government deny behind assault trump previously appear disincline intervene syrian leader attack raise expectations might ready adopt tougher-than-expected stance russia assad main backer u.s. secretary state rex tillerson due travel moscow week spokeswoman may say two leaders agree conversation visit opportunity make progress toward solution prime minister president agree window opportunity exist persuade russia alliance assad long strategic interest spokeswoman say agree u.s. secretary state tillerson visit moscow week provide opportunity make progress toward solution deliver last political settlement spokeswoman say two leaders also stress importance international community include china put pressure north korea constrain threat pose may trump agree russia break tie assad uk pm 's office