Britain plans to participate in a U.S.-led effort to train Syrian opposition troops, according to people familiar with the matter, but it will stop short of joining its closest ally in conducting airstrikes against Islamic State in Syria.

The U.K. government will announce in the coming days its role in training moderate Syrian opposition forces in neighboring countries with the aim of bolstering their ability to fight the extremist group, the people said. Turkey and Jordan will be involved in the plan, one of the people said.

The expected announcement marks an escalation of Britain’s involvement in the international effort against Islamic State after its moves to join airstrikes against the militant group in Iraq and carry out aerial surveillance missions over Syria. Some British politicians, though, say the U.K.’s military response hasn’t gone far enough and that Islamic State, also known as ISIS or ISIL, can only be defeated by tackling the extremists with airstrikes in Syria as well as Iraq. […]