LONDON — The alleged plot to kill Saudi Arabia’s ambassador to the U.S. marks an escalation of Iran’s sponsorship of terrorism overseas, Britain’s foreign secretary said Thursday, as Europe and others discussed their response.

William Hague told lawmakers that Britain was working with the U.S., Saudi Arabia and the European Union to agree on tough international action over the purported plan to assassinate Adel Al-Jubeir in a bombing at his favorite Washington restaurant.