MANCHESTER, England—Russia's support of Syrian President Bashar al-Assad would ultimately hurt the fight against Islamic State, U.K. Foreign Secretary Philip Hammond said, underscoring the complications of Russia's military involvement in the Syria crisis.

Mr. Hammond, speaking Sunday at the main annual conference of British Prime Minister David Cameron's Conservative Party, said the U.K. was open to working with Russia in Syria, but that Russian President Vladimir Putin's government couldn't fight Islamic State but support Mr. Assad. Russia recently began striking targets in Syria in support of Mr. Assad's government, marking a major escalation in its involvement in the crisis.

"Russian support for him will drive the opposition in Syria into the arms of ISIL, strengthening the evil that Putin says he wants to defeat," Mr. Hammond said, using an alternative name for Islamic State. He said the U.K. would continue to lead the push in Europe for maintaining tough sanctions on the Russian government for its aggression in Ukraine.

Russia says its actions in Syria are aimed at fighting terrorists and preserving the country's territorial integrity.

The Conservative Party conference, in Manchester, marks the first time the leadership has addressed party members since Mr. Cameron won a general election in May and achieved the center-right party's first parliamentary majority in 23 years.

But Mr. Cameron and his government faces many challenges, including the situation in Syria and Iraq. Earlier Sunday, Mr. Cameron said Russia's decision to take military action in Syria to support Mr. Assad was a "terrible mistake."

"It's going to make the region more unstable; it will lead to further radicalization and increased terrorism." Mr. Cameron told the British Broadcasting Corp. "I would say to them change direction, join us in attacking ISIL but recognize that if we want to have a secure region, we need an alternative leader to Assad."

Another key issue for Mr. Cameron is Europe, an area where his party is deeply divided. He has promised British voters to secure concessions ahead of a referendum on whether the U.K. should remain in the European Union. He has said he wants Britain to remain part of a reformed EU but some in his party want an exit.

Mr. Cameron and his treasury chief, George Osborne, also have the politically tricky task of finding further areas to cut government spending to fulfill a pledge to continue to reduce the U.K.'s budget deficit. Thousands of people gathered in Manchester on Sunday for a march against the Conservative Party's austerity policies.

Mr. Osborne is due on Monday to announce plans to boost investment in Britain's infrastructure, according to aides. The Treasury chief plans in his speech to say that the government will spend an extra £ 5 billion on projects in the next five years on top of the £ 100 billion already planned, and will set up a National Infrastructure Commission to advise the government on what needs to be built. He will also announce plans to pool the pension savings of British local government workers into half a dozen "wealth funds," which will allocate a portion of their assets to infrastructure investment.

Jason Dean contributed to this article.

Write to Jenny Gross at jenny.gross@wsj.com

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(END) Dow Jones Newswires

October 04, 2015 20:55 ET (00:55 GMT)