MOSCOW (AP) — As Russian President Vladimir Putin marks two decades in power , he boasts about his achievements but remains coy about his political future — a reticence that fuels wild speculation about his intentions.

Putin points to the revival of Russia's global clout, industrial modernization, booming agricultural exports and a resurgent military as key results of his tenure that began on Dec. 31, 1999. On that day, Russia's first President Boris Yeltsin abruptly stepped down and named the former KGB officer his successor, paving the way for his election three months later.

Critics accuse Putin of rolling back post-Soviet freedoms to establish tight control over the political scene, marginalize the opposition and stifle critical media. They hold him responsible for tensions with the West after Russia's 2014 annexation of Ukraine's Crimea, which bolstered his approval ratings but triggered U.S. and European sanctions.

“Putin stopped the normal development of Russia as a normal market economy and a normal political democracy" and turned the country into a “global spoiler,” said Andrei Kolesnikov, a researcher with the Carnegie Moscow Center.

Kremlin watchers are trying to predict what will happen after Putin's current six-year term ends in 2024. They agree on one thing: Putin, Russia's longest-serving leader since Soviet dictator Josef Stalin, will likely stay at the helm.

A fitness fan, the 67-year old Putin appears in good shape to stay on. He regularly practices judo, skis and plays ice hockey in a demonstration of his vigor.

He remains widely popular, although the propaganda effect of Crimea's annexation has worn off amid stagnant living standards, a rise in the retirement age and other domestic challenges.

Putin can easily use the rubber-stamp parliament to scrap term limits, but most observers expect him to take a less straightforward approach. A law faculty graduate, the Russian leader prefers more delicate methods that have a democratic veneer.

Earlier this month, Putin hinted at possible constitutional amendments to re-distribute powers among the president, the Cabinet and parliament.

He didn't specify what changes could be made, but the announcement may signal his intention to trim presidential powers and continue ruling the country as prime minister.

There are other opportunities. Kazakhstan's longtime leader Nursultan Nazarbayev offered an example this year when he abruptly resigned and had his protege elected president in a snap vote. The 79-year-old Nazarbayev retained his grip on power by securing a prominent position as head of the nation's security council.

There is another, more dramatic option. Many in neighboring Belarus fear that the Kremlin could push for a full merger of the two ex-Soviet allies to allow Putin to become the head of a new unified state.

When asked recently if he was considering it, Putin dodged the question.

Each of those potential options carries major risks.

Putin moved into the prime minister's seat from 2008-2012 after eight years as president to observe a constitutional limit of two consecutive terms, allowing Dmitry Medvedev to take the top seat.

Putin continued calling the shots under Medvedev, who obediently stepped down after one term. Putin benefited from his placeholder's move to extend the presidential term to six years, but still wasn't quite happy with the “tandem rule.”

Putin was particularly critical of Medvedev's decision to let the United Nations give the go-ahead to a 2011 Western air campaign in Libya that helped oust longtime dictator Moammar Gadhafi and plunged the country into chaos.

And at home, the announcement of Putin's return to the presidency sparked massive protests in Moscow in 2011-2012 and caused a rift among elites. Putin's aides suspected some of Medvedev's lieutenants of prodding their boss to stay for a second term and encouraging the protests.

Putin's statement this month about a possible change to the constitution to limit the president to just two terms altogether was widely interpreted as a signal that he was contemplating creating a new governing position for himself while trimming the authority of his successor.

If Putin chooses to become prime minister with new broad powers, it may raise other threats.

By empowering a parliamentary majority to name the prime minister, Putin would become more vulnerable because he will depend on the ruling party's performance. While Putin's approval ratings have remained high, the popularity of the main Kremlin-directed party, the United Russia, has plummeted and the president has kept it at a distance.

A merger with Belarus to create a new leadership position has even greater risks. The prospect may excite some Russians who dream about revival of imperial glory, but it is certain to trigger strong resistance in Belarus and further antagonize the West.

Belarus' authoritarian President Alexander Lukashenko, who has been in office for more than a quarter-century, has vowed to uphold Belarus' post-Soviet independence. While the Kremlin has pressured him by raising energy prices and cutting subsidies, Lukashenko has remained adamant and even warned recently that Russia's attempt to take over his country could trigger a war with NATO.

“The interest of Lukashenko is to be the dictator of his own nation state, not the person who will be dependent on the will of Putin,” Kolesnikov said.

Whatever path Putin chooses, he's widely expected to keep his intentions secret until the last moment.

“This uncertainty has its advantages — you can play groups of interests against each other, you can hold them in this situation of uncertainty," said Moscow-based political analyst Yekaterina Shulman. “But it can’t go on for too long because it provokes infighting within the elites.”

She noted that the Kremlin may call the parliamentary elections that are currently set for 2021 at an earlier date before approval ratings plummet.

“It’s important to have a loyal majority in the parliament," Shulman said. “How to achieve this is a tricky question.”

Shulman argued that the Kazakhstan-style scenario appears the most likely. She said staying at the helm but sharing authority with his successor would allow Putin to temper an inevitable succession battle among his lieutenants.

“The difficulty in the successor model is that the whole amount of power vested in the current president is untransferable indeed to any other person," Shulman said. “But if this power is redistributed, at least part of it, then it’s easier for the decision-makers to agree on the figure of the potential successor.”

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Konstantin Manenkov in Moscow contributed to this report.