WASHINGTON — Secretary of State John Kerry will fly to London to meet with his Russian counterpart in a 11th-hour effort to persuade the Kremlin not to move forward with the annexation of Crimea, American officials said on Wednesday.

Mr. Kerry’s meeting with Sergey V. Lavrov, the Russian foreign minister, will come on Friday, just two days before a Sunday referendum in Crimea in which the region is to decide whether to become part of Russia.

The United States and its partners have said that such a referendum would not be legal, and Mr. Kerry has warned that Russian steps to annex the region would “close any available space for diplomacy.”

Mr. Kerry announced the meeting with Mr. Lavrov in testimony to the House Appropriations Subcommittee on Foreign Operations. His testimony came as Ukraine’s acting prime minister, Arseniy Yatsenyuk, was scheduled to consult with Mr. Kerry at the State Department this afternoon before meeting with President Obama and Mr. Kerry at the White House.