Russia has reportedly been moving artillery units to parts of northern Syria populated by President Bashar al-Assad’s forces, a move that could indicate that Russia is readying to bolster its ally should full-scale fighting resume.

The Wall Street Journal, citing U.S. officials, reported that some Iranian forces have also returned to areas controlled by Syrian government forces. "Hundreds" of Iranian troops have been witnessed in Syria, according to a report in Fox News, which also indicated that Russia has relocated the bulk of its weapons, including rocket-propelled artillery, near Aleppo.

Officials reportedly began detecting the artillery movements approximately two weeks ago.

Russia and Iran, both allies of Assad, have propped up the controversial leader’s regime as it has clashed with opposition groups.

The latest report comes after another round of U.N.-sponsored peace talks between Assad and opposition groups faltered. The groups have abided by a partial ceasefire deal brokered by the U.S. and Russia in February. Russia’s movement of artillery signifies that the ceasefire could soon falter.

The ceasefire agreement was the subject of an "intense" phone call between President Obama and Russian President Vladimir Putin on Monday, White House press secretary Josh Earnest told reporters earlier this week. Obama urged Putin to encourage Assad to abide by the partial ceasefire and halt attacks on opposition forces.

The conversation also touched on Russia’s intervention in Ukraine, though Obama did not broach Russia’s recent military provocations against the U.S. in the Baltic Sea.

Russia intervened militarily in Syria last September under the pretext of combatting ISIS, the terror group behind the attacks in Paris and Brussels. However, many Russian airstrikes targeted areas controlled by opposition groups fighting the Syrian government forces.

Last month, Putin ordered the main part of his forces to begin withdrawing from Syria, saying that "the task that was assigned to the Ministry of Defense and the armed forces as a whole has achieved its goal." Days later, the Russian president noted in remarks at a Kremlin ceremony that Russia’s military capability in Syria could again be built up "literally within a few hours."