The “strength of the support” shown to the UK over the poisoning of former Russian agent Sergei Skripal and his daughter has been encouraging, the foreign secretary has said.
Boris Johnson said the UK had been talking to friends and there had been a “willingness” to show “solidarity”.
Russia’s foreign minister said claims of Russian involvement were “rubbish”.
The UK has given Russia a midnight deadline to explain why a Russian-made nerve agent was used in the attack.
Russian Foreign Minister Sergei Lavrov said that Russia had been refused access to the substance that was used to poison Mr Skripal and it would not respond to the ultimatum until it was given access.
The UK ambassador to Russia, Laurie Bristow, has been summoned to the Russian Foreign Ministry, according to Russian news agency Interfax.
Former double agent Mr Skripal, 66, and his daughter, Yulia, 33, were found slumped on a bench in Salisbury city centre in Wiltshire on 4 March. They remain in a critical but stable condition in hospital.
Det Sgt Nick Bailey, who fell ill attending to the pair, remains seriously ill, but has been talking to his family.
Nato secretary-general Jens Stoltenberg said: “The UK is a highly valued ally, and this incident is of great concern.”
US Secretary of State Rex Tillerson said it appeared the “really egregious act… clearly came from Russia” and there should be “serious consequences”.