The Russian government on Tuesday criticized arms deliveries from Great Britain to Ukraine as "extremely dangerous".
The British action "does not contribute to reducing tensions," said a spokesman in Moscow.
Earlier, the British government announced that it had started supplying anti-tank weapons to the Ukrainian army.
In addition, a "small number" of British soldiers will teach the Ukrainian armed forces how to use the systems, Defense Secretary Ben Wallace said on Monday evening in the House of Commons.
Jochen Buchsteiner
Political correspondent in London.
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Peter Carstens
Political correspondent in Berlin
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Wallace emphasized that Ukraine's "defense capability" should be improved and spoke of "Russia's increasingly threatening behavior".
However, "they are not strategic weapons and they do not pose a threat to Russia," he said.
The Labor Party showed support.
However, Great Britain must ensure that "anti-tank weapons are only used if Russia attacks".
In NATO circles, the Russian troop transfers to the Ukrainian border are viewed with growing unease and fears of an attack.
Reports that Germany had refused an overflight to a Royal Air Force plane carrying the first weapons were denied on Tuesday.
The C-17 Globemaster machine did not take the shortest route over German airspace on Monday, but flew over the North Sea, Denmark and then Poland.
Assumptions in networks that the overflight had been denied were rejected by both British and German authorities.
According to the information, there had been no request from the British side for an overflight, a so-called "diplomatic clearance", so none was rejected.
A signal to the Kremlin
Maps of flight movements on the Internet later showed that both American and British reconnaissance aircraft took off from England in order to collect data in and around the Ukraine after crossing German airspace. According to information from this newspaper, military aircraft from allied states such as America, Great Britain or France usually do not require such a permit for their everyday flights via Germany or to US bases in Germany. The "diplo clearance" procedure is used when foreign military aircraft, such as the presidential aircraft of another country, want to cross the airspace of third countries. Granting such approval is routine, but it can also be used diplomatically in conflict situations and then denied.
The Minister of Defense left it open which weapons systems Great Britain will supply – and how many. Experts suspected on Tuesday that it was a new generation of light defensive weapons with a range of a few hundred meters. They are similar to the Javelin-type anti-tank weapons that the United States has been supplying to Ukraine since 2018, but are not quite as effective. So far, London had only sold ships and naval equipment to Ukraine. A contract worth almost two billion euros provides for the delivery of two minesweepers and the joint construction of eight ships for the Ukrainian Black Sea Fleet.
The British security establishment has been lobbying for a robust response to Russia for many weeks. How far London is willing to go in the event of a Russian intervention in Ukraine is left open. However, Wallace said he did not want to encourage "false hopes" that the British government was sending armed soldiers into the country. Asked by a Conservative MP what further support NATO plans to provide in the event of Russian intervention, Wallace replied that he hoped the arms shipments would send a signal to the Kremlin. "I hope so because President Putin has not yet made a final decision." Wallace announced that he would invite his Russian counterpart Sergei Shoigu to London for bilateral talks. "We are ready,to discuss the matters related to mutual security concerns and to engage constructively and in good faith," Wallace said.