NATO also fears Putin may have designs on Transdniestria, a part of another former Soviet republic, Moldova.
Russia has said it is complying with international agreements on troop movements and has no plans to invade.
(Read more: Russian troops seize Ukrainian naval base)
In what has become the biggest East-West confrontation since the Cold War, the United States and the European Union have imposed personal sanctions on some of Putin's closest political and business allies. But they have held back so far from measures designed to hit Russia's wider economy.
Obama also discussed the crisis at a meeting in The Hague with Chinese President Xi Jinping, who has voiced support for Ukraine's sovereignty but refrained from criticising Russia.
The West wants Beijing's diplomatic support in an effort to restrain Putin but while Xi called for a political solution, he did not harden China's position towards Moscow.
Russia formally annexed Crimea on March 21, five days after newly-installed pro-Moscow regional leaders held a referendum that yielded an overwhelming vote to join Russia. Kiev and the West denounced the annexation as illegal.
In one sign of a possible easing of tension, Lavrov agreed to hold a first meeting with his Ukrainian counterpart, Andriy Deshchytsya, on the sidelines of the nuclear security summit.
The first 50 out of 100 observers dispatched by the pan-European OSCE security watchdog arrived in Ukraine on Monday to monitor potential trouble spots and defuse tensions. Russia relented late last week and agreed on a mandate after prolonged wrangling, but the monitors will not be allowed to enter Crimea.
(Read more: Why sanctions and S&P may spoil Russia's Crimea party)
Western officials are now focused less on persuading Putin to relinquish Crimea - a goal that seems beyond reach - than on deterring him from seizing other parts of Ukraine, which was under Moscow's control within the Soviet Union until its break-up in 1991.
Persuading Europeans to sign on to tougher sanctions could be difficult. The EU does 10 times as much trade with Russia as the United States, and is the biggest customer for Russian oil and gas. The EU's 28 members include countries with widely varying relationships to Moscow.
But German Chancellor Angela Merkel, the EU's most powerful leader, has taken a tough line with Putin and supported EU moves to reduce the bloc's long-term dependence on Russian energy.
Despite the disruption to East-West relations, Washington wants other diplomatic business with Moscow to continue. U.S. Secretary of State John Kerry held talks with Lavrov after meeting the head of the Organisation for the Prohibition of Chemical Weapons, overseeing the destruction of Syria's toxic stockpile in action sponsored jointly by Washington and Moscow.
(Read more: Ukraine's battered economy)
Russia hit back symbolically at Canada, announcing personal sanctions against 13 Canadian officials in retaliation for Ottawa's role in Western sanctions so far. Moscow has already taken similar measures against senior U.S. Congress members but not yet European officials.