The statement came from a top US State Department official on Tuesday night, the Reuters news agency reported.
The official said: "We have made clear that there will be increased consequences. We have ... a number of tools."
Earlier in the day, in an apparently coordinated move, Russian-installed authorities in occupied areas of four Ukrainian regions, Donetsk, Luhansk, Kherson and Zaporizhzhia, unveiled plans to stage referendums on joining Russia on September 23-27.
These regions constitute some 15 percent of Ukrainian territory, an area roughly the size of Hungary, according to Reuters.
Putin orders partial mobilisation
On Wednesday, Russian President Vladimir Putin announced he had signed a decree on partial mobilisation for his country's military campaign in Ukraine, saying his aim was to defend Russian territory.
In a televised address, Putin accused the West of engaging in "nuclear blackmail" against Russia and of seeking to destroy his country, news outlets reported.
The Russian leader also said his aim was to "liberate" eastern Ukraine's Donbas region, comprising Donetsk and Luhansk, claiming that most people in areas under Russian control did not want to be ruled by Kyiv, as reported by Reuters.
Moreover, Putin warned that Russia had "lots of weapons to reply" to what he described as Western threats, adding that he was not bluffing, Reuters reported.
World leaders slam Russia over invasion of Ukraine, referendum push
Meanwhile, world leaders meeting at the United Nations General Assembly (UNGA) in New York, including Polish President Andrzej Duda, condemned Russia's invasion of Ukraine and the referendum plan.
French President Emmanuel Macron told reporters that if the plan "wasn't so tragic we’d laugh," news.sky.com reported.
German Chancellor Olaf Scholz said in his UN address that Putin would only give up his "imperial ambitions" that risk destroying Ukraine and Russia if he recognised he couldn’t win the war.
"This is why we will not accept any peace dictated by Russia and this is why Ukraine must be able to fend off Russia's attack," Scholz added.
Japan’s Prime Minister Fumio Kishida told the UN General Assembly that, with Russia being a permanent member of the UN Security Council, its invasion of Ukraine put the United Nations’ credibility “in danger.”
"Russia's invasion of Ukraine is a conduct that tramples the philosophy and principles of the UN charter ... It should never be tolerated," Kishida said.
Meanwhile, Ukraine’s Foreign Minister Dmytro Kuleba told reporters at the UN when asked about Russia's referendum plans: "The Russians can do whatever they want. It will not change anything."
He added in a tweet: "Ukraine has every right to liberate its territories and will keep liberating them whatever Russia has to say."
‘Initiative at the front is with Ukraine’: Zelensky
Ukrainian President Volodymyr Zelensky said in a video address released early on Wednesday that "the situation at the front clearly indicates the initiative is with Ukraine."
Moreover, Ukraine's position did not change because of "some noise" from Russia, Zelensky said in reference to the referendums.
The referendum plans, and Putin’s order on partial mobilisation, came amid Ukraine’s major, successful counteroffensive in the east and south of the country.
Zelensky's chief of staff, Andriy Yermak, commented on Tuesday: "Naive blackmail, [and] threats to hold referendums and announce mobilisation from those [Russians] who only know how to fight with children and unarmed people. This is what the fear of defeat looks like. Russia is afraid, it tries to primitively manipulate.”
Justification for full mobilisation?
Meanwhile, the Institute for the Study of War (ISW) has assessed that the Kremlin’s "annexation plans are primarily targeting a domestic audience; Putin likely hopes to improve Russian force generation capabilities by calling on the Russian people to volunteer for a war to ‘defend’ newly claimed Russian territory.”
Moreover, reframing the battles in occupied territory as an assault on Russia could give the Kremlin a justification to mobilise its 2 million-strong military reserves, Reuters reported.
The partial mobilisation announced by Putin on Wednesday will see 300,000 reservists called up and would apply to those with previous military experience, according to Russia's Defence Minister Sergei Shoigu.
Wednesday is day 210 of the Russian invasion of Ukraine.
Source: PAP, Reuters, news.sky.com, understandingwar.org