Russian President Vladimir Putin has ordered his military to increase the size of the country’s armed forces by 137,000 amid Moscow’s invasion of Ukraine.

Mr Putin’s decree signed on Thursday did not explain whether the military will increase its ranks by drafting a bigger number of conscripts, increasing the number of volunteer soldiers or using a combination of both.

The presidential decree seeks to boost the number of Russian armed forces’ personnel to 2,039,758 overall, including 1,150,628 servicemen.

A previous order put the military’s numbers at 1,902,758 and 1,013,628 respectively at the start of 2018.

The Kremlin has said that only volunteer contract soldiers take part in what it calls the “special military operation” in Ukraine, rejecting claims that it was considering a broad mobilisation.

Russian media and non-governmental organisations say Russian authorities have sought to bolster the number of troops involved in the military action in Ukraine by attracting more volunteers, engaging private military contractors and even offering amnesty to some prisoners in exchange for a tour of military duty.

All Russian men aged 18-27 must serve one year in the military, but a large share avoid the draft for health reasons or deferments granted to university students. The share of men who avoid the draft is particularly big in Moscow and other major cities.

The Russian military rounds up draftees twice a year, during the spring and in the fall. Putin ordered the drafting of 134,500 conscripts during the latest spring draft.

In recent years, the Kremlin has emphasised increasing the share of volunteer contract soldiers as it sought to modernise the army and improve its readiness.

Before the Kremlin sent troops into Ukraine, the Russian military had over 400,000 contract soldiers, including 147,000 in the ground forces.

Military observers have noted that if the campaign in Ukraine drags on, those numbers could be clearly insufficient to sustain the operations in Ukraine, which has declared a goal of forming a million-strong military.