LONDON—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. 

Prime Minister David Cameron has repeatedly vowed to destroy the militant group, arguing that it poses a threat to security in Britain. Adding to the pressure on him to respond are the beheadings of two kidnapped Britons by Islamic State, which is believed to be holding at least one other British man captive. 

But public opinion in the U.K. is divided over whether the British military should join the U.S. in bombing in Syria, especially after the loss of British soldiers’ lives in campaigns in Iraq and Afghanistan. Mr. Cameron suffered an embarrassing parliamentary defeat on the issue in 2013 when he sought to secure parliament’s backing for launching airstrikes in Syria in response to claims that Syrian President Bashar al-Assad used chemical weapons. 

Mr. Cameron faces a general election in May in which his Conservative Party is in a close-fought race with the opposition Labour Party, which voted against Mr. Cameron in the 2013 vote on Syrian intervention. If the Conservatives win the May 7 election, it will ask Parliament to approve joining the U.S. and its allies in carrying out airstrikes in Syria, U.K. Defense Secretary Michael Fallon, a Conservative lawmaker, said in an interview with The Wall Street Journal earlier this month. “The prime minister has always been clear that ISIL has to be defeated in both countries,” he said. 

Mr. Cameron’s office noted the prime minister’s previous comments, in which he said British military intervention in Syria wouldn’t happen without a vote by U.K. lawmakers. 

Mr. Fallon said that the U.S. and Britain had been looking at a number of possible locations to carry out the training of Syrian opposition troops and that three or four countries had stepped forward, but declined to specify which. He said that Britain was considering sending fewer than 100 military trainers. 

He added that the effort presented challenges, saying the process had been frustratingly slow. “We have to be sure that the people we’re training are the right people who are going to carry the fight to ISIL. Syria is a very complicated battleground,” said Mr. Fallon. He said the first task was to train them to defend their own towns and villages “and then see what forces we can take out of the fight in Syria to train and send back into Syria.” 

In September, when Mr. Cameron secured parliamentary approval for launching airstrikes against Islamic State in Iraq, the prime minister said there was a strong case for Britain to do more in Syria. But he said that he didn’t think there was enough political support in the U.K. for military intervention and that he thought the situation in Syria was more complicated than Iraq because of the country’s civil war and the Assad regime. 

Michael Clarke, director general of Royal United Services Institute, a London-based defense think tank, said Western nations determined to avoid committing troops to fight Islamic State have been playing for time while regional forces organize the boots on the ground. But the more gains that are made against Islamic State in Iraq, the greater the pressure will be for Britain to join the military effort in Syria, particularly since the Royal Air Force has capable aircraft and ordinance to deliver precision airstrikes. 

“It may be that a future [U.K.] government says we just don’t have the stomach to take it on, but it will look worse and worse in the eyes of the Americans and the rest of the coalition if the Brits are playing a very reticent role,” he said. 

Gen. Sir Peter Wall, who was the head of the British Army until last year, said Britain and its allies had to be careful not to exacerbate the situation in the Middle East and turn what has been a struggle among Islamist groups into something that made the West the focus for extremists. 

“We have a role in the periphery of the Middle East supporting, containing, perhaps training and assisting indigenous forces to do the job themselves,” he said at the think-tank panel discussion. 

The Wall Street Journal