MOSCOW — Russian President Vladimir Putin, his popularity driven up by the crisis in Ukraine, is firmly on course to win a new term in 2018 if he decides to run, an independent polling agency said on Thursday.

A survey by the Levada research group found 81 percent of respondents would be ready to vote for Putin if a presidential election took place now, up from 77 percent in April and 68 percent in January.

Putin's public support has been rising since the start of the Ukraine crisis, during which Russia has annexed the Crimea region, the president has fiercely criticized the pro-Western leadership in Kiev and relations with the West have worsened.

"His determination appeals to the majority of the electorate," Alexei Grazhdankin, deputy head of Levada, said in a statement. "It is unlikely that by 2018 a politician will emerge who can compete with Putin for the presidential seat."

Putin's approval rating, a separate monthly index published by Levada, exceeded 70 percent for most of his first two terms from 2000 to 2008, when Russia benefited from an oil-fueled economic boom. At times it rose above 80 percent.

Putin won a six-year third term as president in March 2012 with almost 64 percent of the votes, despite street protests against him in the preceding months.

Opinion polls show the former KGB spy remains Russia's most popular politician. He has not ruled out seeking a six-year fourth term in 2018.

The crisis in Ukraine has developed into the worst standoff between Russia and the West since the end of the Cold War, prompting Washington and Brussels to impose sanctions on some Russian individuals and companies.