The United States and Ukraine signed a joint agreement here Monday designed to stem the threat of bioterrorism by placing modern safeguards on deadly pathogens and other material dating from a Soviet-era biological weapons program that now could be vulnerable to theft.

"The agreement has a benefit for the citizens of both countries," said Sen. Richard Lugar, R-Ind., chairman of the Senate Foreign Relations Committee, who has been working several years to achieve the U.S.-Ukraine accord.

As Lugar and Sen. Barack Obama, D-Ill., met with Ukrainian leaders and participated in a signing ceremony for the biological weapons agreement, the Russian Foreign Ministry issued a rare written apology to the senators for detaining them more than three hours Sunday as they tried to leave Russia for Ukraine.

There was no immediate explanation for the delay, but Moscow officials agreed to meet with their U.S. counterparts to discuss why American planes have repeatedly encountered difficulties leaving Russia. The ministry said the U.S. plane technically had not been detained, but a spokesman added, "We regret the misunderstanding that arose and the inconvenience caused to the senators."

Lugar did not dwell on the plane incident after leaving Russia and arriving in Kiev. Instead he sought to draw attention to the freshly minted agreement that effectively expands the Nunn-Lugar Act of 1991 to allow the United States to help protect Ukraine's biological weapons.