MOSCOW (AP) — Sergei Ivanov, a former Russian defense minister once seen as a possible successor to President Vladimir Putin, has died. He was 73.

The Kremlin said Ivanov died on Friday without providing the cause of death or giving other details. Putin sent his condolences to Ivanov's family.

Putin and Ivanov knew each other since the 1970s when they both worked as young KGB officers in their hometown of Leningrad, as St. Petersburg was called during the Soviet times.

When Putin was first elected in 2000, Ivanov served as secretary of Russia's Security Council. In 2001, Putin made Ivanov his defense minister, the job he held until 2007, overseeing the second war in Chechnya that crushed the region's separatist bid.

When Putin decided to step down due to term limits and shift into the prime minister's seat in 2008, Ivanov was widely viewed as his most likely successor. However, Putin picked another longtime associate, Dmitry Medvedev, to serve as his placeholder until reclaiming the presidency in 2012. Some observers argued that Putin ditched Ivanov's candidacy because he saw Ivanov as overly ambitious and feared that he could try to hold onto the presidential seat.

Ivanov remained at Putin's side as deputy prime minister from 2007-2011, and then served as the Kremlin chief of staff from 2011-2016.

In 2016, Ivanov was named a presidential envoy for environment protection and transport, a job that carried no political weight and was widely seen as a honorary retirement. He stepped down earlier this year.

Along with other Russian top officials, Ivanov has been targeted by the U.S. and the EU sanctions in response to Moscow's military action in Ukraine.