World leaders must do more to hold Russia and Syria to account for their use of chemical weapons, Theresa May has warned as she heads to New York for two days of talks at the United Nations.

The “despicable” attacks in Salisbury and rebel-held Syrian areas recently are a threat to the international system, the Prime Minister said.

Action must be taken to prevent further use of chemical weapons as well as deal with previous incidents, she added.

The PM will issue a warning when she addresses the UN Security Council on Wednesday.

On the first day of her visit, she will hold a series of meetings with international counterparts, including Iran’s President Hassan Rouhani, and will raise the case of charity worker Nazanin Zaghari-Ratcliffe, who has been jailed in Tehran on spying charges.

Mrs May is also hosting an event with Emmanuel Macron on improving education for girls. The French President last week called Brexit campaigners “liars” and said leaving the bloc was “not without costs” as EU leaders rejected the PM’s Chequer’s plan.

A meeting with US President Donald Trump has been scheduled for Wednesday evening and discussions will also be held with businesses including Google.

Britain accused Russia of “obfuscation and lies” after the prime suspects in the nerve agent attack on former spy Sergei Skripal and daughter Yulia claimed they visited the UK as tourists.

Salisbury incident
Alexander Petrov (left) and Ruslan Boshirov were identified as the suspects in the Salisbury nerve agent attack (PA)

Alexander Petrov and Ruslan Boshirov were identified by the UK as members of the GRU, Russia’s military intelligence service, but made widely mocked claims that they only made the trip in early March to visit the “wonderful town” of Salisbury and its famous cathedral.

Speaking ahead of the UN General Assembly (UNGA) visit, Mrs May said: “As we meet in the UN Security Council this week the red lines around the use of chemical weapons are being eroded.

“The Syrian regime has repeatedly used these appalling weapons against its own people while the Russian state has deployed them on UK streets.

“Attacks such as Salisbury and Ghouta are despicable in their own right, but they are also a threat to the wider international system.

“Each time we fail to challenge the use or development of weapons of mass destruction (WMD) it erodes the framework of treaties we have built up so painstakingly over the past few decades.

“We worked closely with our allies on a co-ordinated response to Russia’s use of chemical weapons in Salisbury, resulting in 28 countries as well as Nato joining us in expelling a total of over 150 Russian intelligence officers, the largest collective expulsion ever.

“But the international community needs to do more together, both to prevent future chemical weapons use and to ensure those who use them are held to account, but also to tackle the range of other threats to global security, including the proliferation of WMD.”

Foreign Secretary Jeremy Hunt and International Development Secretary Penny Mordaunt are also attending UNGA.

Ms Mordaunt announced new protections for vulnerable children at risk of falling prey to traffickers.

A Unicef project backed by the UK will provide up to 400,000 girls and boys in Ethiopia, Somalia and Sudan at risk of slavery with birth registration documents to help stop them being dragged into forced labour and under-age marriage.