US Secretary of State John Kerry arrived in Moscow on Tuesday aiming to narrow the gap with the Kremlin on international efforts to advance Syria peace talks and make a political transition in the war wracked country.

Kerry is scheduled to talk with his Russian counterpart Sergei Lavrov before the two inform Putin - Russia's real decision maker - what was discussed.

The outcome of the talks is likely to determine whether international talks will advance to the next stage or face further hurdles and setbacks. A positive outcome from Tuesday's meeting could mean international talks as soon as later this week.

On the agenda are a number of issues that have until now remained intractable in resolving a conflict that involves a criss-cross of countries, opposition groups and rebel factions with different interests and views.

Differences over Assad

The role of Syrian President Bashar al-Assad in any political transition tops the list of differences. Russia argues that the Syrian people should be the ones to decide the future of Assad and that his regime, backed by Russia, should play a role in international efforts to squash the "Islamic State" (IS) militant group.

A meeting of Syrian opposition and rebel groups in the Saudi capital of Riyadh last week agreed for the first time on a unified position and will meet with representatives of the Assad regime as early as January, but insisted that Assad play no role in a political transition.

However, Russia has noted grievances with the Saudi meeting, saying that the grouping of some 100 opposition and rebel delegates was not representative of the Syrian people and that it includes terrorist groups.

The Riyadh meeting ran parallel to another unfinished process, whereby it was agreed that Jordan would work on drawing up a list of legitimate Syrian opposition actors that could be involved in negotiations, and another list of terrorist organizations to be excluded.

The Assad regime has shunned talking to foreign-backed rebel groups deemed as terrorists. Still, the United States hopes that Russia can use its leverage in Syria to soften Assad's position and, ultimately, help ease him out of power and usher in a political transition.

The United States and the West have softened their stance on the future of Assad as frustration builds over the course of the Syrian conflict - that after five years has led to a stalemate on the battlefield, a flood of refugees and the rise of the "Islamic State" (IS).

The United States has said that Assad could play a possible role in a political transition - during which there would be a ceasefire with groups deemed legitimate - but that he must ultimately step down from power.

But it still remains for the United States to convince its allies, most notably Saudi Arabia, Turkey and Qatar - all key financial and military backers of the Syrian opposition - who demand Assad step aside before any political process gets underway. Barring that, they have threatened to up their support to Syrian rebels until the regime is ousted.

Focus on IS

Parallel to the political talks is the issue of pushing back IS. Russia would like the United States and other anti-IS coalition partners to join it against the extremist group, but the United States has so far refused to coordinate.

The US and its allies argue Russia is primarily targeting moderate Syrian rebel factions in an effort to bolster the Assad regime. Russia denies the allegations.

The United States would ultimately like to cobble together the moderate stripes of the Syrian opposition to take the ground fight IS. In order for this to happen, a political transition and cease-fire between the Syrian regime and rebels would be necessary.