What next as Russia's military offensive stalls

The Kremlin has declined to comment on details of the military operation, and its Defense Ministry is adamant that the campaign has been successful.

A senior US defense official said there were indications that in the last 24 hours Moscow had become frustrated by slow progress due to the unexpectedly strong Ukrainian defense and failure to achieve complete air dominance.

The UK's Ministry of Defense tweeted that Russian military progress was stalling.

The Institute for the Study of War, a Washington-based think tank, notes that Ukrainian resistance remains highly effective, despite the Russians' huge advantage in firepower.

"Actually, the Pentagon generals believed that in 72 hours, the Ukrainian organized resistance would collapse and then it would be maybe become guerrilla warfare. So they were providing actual weapons before the war broke out — weapons that can be used by guerrillas such as shoulder-launched stingers, and javelins, and other anti-tank weapons," Pavel Felgenhauer, a Moscow-based defense analyst, told DW.

Russian surprise at Ukraine's resistance

Analysts say that the Russian military leadership has been taken by surprise by the lack of impact its aerial campaign is having. "The Russian forces have a hard time suppressing the air defense and aerospace forces of Ukraine and Ukrainian forces," Mathieu Boulegue, a research fellow with the Russia and Eurasia Program at Chatham House, a London-based think tank, told DW.

The Pentagon believes that Russia has currently moved at least 50% of its forces into Ukraine out of an estimated 150,000 troops amassed at the border.

It's not just the regular army making inroads against the Russian forces but also volunteers who have formed militias and are arming themselves with home-made Molotov cocktails, setting up road blocks and removing road signs to confuse Russian troops.

Most analysts predict that the current war phase is just the first stage of a multi-pronged tactic and that the Russian military will now broaden the campaign.

"They began with a kind of hybrid warfare that everyone was expecting. I mean Special Forces, paratroopers penetrating, which are not really classical things that the Russian military is good at. It's not the way were fighting in Syria, where they liberated Syrian cities by turning them into piles of rubble. Now most likely they'll take things more seriously, more Russian-style, and be more aggressive in using heavy weaponry," Felgenhauer said.

Putin's next steps

In the meantime, Putin has reacted to the stalled military campaign and has put Russian nuclear deterrent forces on high alert.

"Senior officials of the leading NATO countries also allow aggressive statements against our country, therefore I order the Minister of Defense and the Chief of the General Staff [of the Russian Armed Forces] to transfer the deterrence forces of the Russian army to a special mode of combat duty," Putin said in the statement, quoted by the state-run TASS news agency.

It's not clear what a "special mode of combat duty" entails at this stage. Former Russian military officer Konstantin Eggert told DW he had "trouble discerning" what Putin meant when he said nuclear forces are on a higher alert as those forces are always on a "heightened alert," he said.

In a further development, there have reportedly been numerous sightings of thermobaric multiple rocket launchers that can be mounted onto the chassis of T-72 tanks.

One such rocket launcher, the TOS-1, was seen near the city of Belgorod near Russia's border with Ukraine on Saturday afternoon, CNN reported.

The TOS-1 is one of the most intimidating weapons systems in Russia's conventional arsenal. It consists of a fuel container and two separate explosive charges. They use oxygen from the surrounding air to generate a high-temperature explosion.

The first charge ignites the container to disperse the fuel in a cloud that then mingles with oxygen in the air. The cloud then flows around objects and into structures. The second charge detonates the cloud, creating a massive shock wave and sucking the air from those in the vicinity.

The weapon was first used by the Soviet military in Afghanistan, and most recently in Syria.

Russian forces have already been using the BM-21 "Grad" ("Hail") multiple rocket system in Ukraine's east and south. Images of destroyed or abandoned BM-21 rocket launchers have circulated from Kharkiv, Ukraine's second-largest city, however there's no independent verification of the sightings at this stage.

There's also concern about an increased use of Kalibr cruise missiles against Kyiv and the southern port city of Odesa. These missiles can be fired from ships, aircraft and submarines and can be equipped with both conventional and nuclear warheads.