An agreement between the United States and Russia to slash their nuclear arsenals was in danger of collapse Tuesday after an influential Republican senator said it should not be voted on this year.

With a terse statement, Sen. Jon Kyl, R-Ariz., dealt a major setback to President Barack Obama's efforts to improve ties with Russia and to his broader strategy for reducing nuclear arms worldwide. The treaty, known as New START, had been seen as one of Obama's top foreign policy accomplishments.