Russian president Vladimir Putin has said that Moscow is helping China build a system to warn of ballistic missile launches.

Since the cold war, only the United States and Russia have had such systems, which involve an array of ground-based radars and space satellites. The systems allow for early spotting of intercontinental ballistic missiles.

Speaking at an international affairs conference in Moscow on Thursday, Putin said Russia had been helping China develop such a system. He added that “this is a very serious thing that will radically enhance China’s defence capability”.

His statement signalled a new degree of defence cooperation between the two former Communist rivals that have developed increasingly close political and military ties while Beijing and Washington have sunk into a trade war.

In June, Chinese president Xi Jinping called Putin his “best and bosom friend,” adding he cherished their “deep friendship.”

There was no immediate comment from Beijing, but Putin’s claim got a mixed reaction online. On China’s Twitter-like social media platform Weibo, some users cheered “new heights in strategic cooperation”, but others questioned whether Beijing needed Russia’s help with military hardware.

“Once again, Russian bragging! Maybe its linked to their national culture,” said one user. “Chinese wouldn’t brag like that”.

Earlier this week China debuted new military hardware including a “hypersonic” missile experts believe could be difficult for the US to counter. The missile, known as the DF-17, can in theory manoeuvre sharply at many times the speed of sound, making it extremely difficult to defend against.

In August China and Russia accused the United States of stoking a new arms race by testing a cruise missile, just weeks after Washington withdrew from a cold-war era missile control treaty that would have barred the test launch.

The ground-launched missile, a conventionally-configured version of the nuclear-capable Tomahawk cruise missile, hit its target after over 500 kilometres of flight during the test, the Pentagon said in a statement.

Ground-launched versions of the missile had been removed from service decades ago, after the intermediate-range nuclear forces treaty was signed by Ronald Reagan and Mikhail Gorbachev in 1987.

The treaty’s ban on missiles with ranges between 500km and 5,500km aimed to reduce the ability of both countries to launch a nuclear strike at short notice.

Russia’s deputy foreign minister, Sergei Ryabkov, condemned the launch, but said Moscow was not looking to start a new arms race, and would not deploy any new missiles unless the US did first.

Beijing also attacked the US, warning that the missile test could lead to “another round of the arms race”, and have a “serious negative impact” on international and regional security.”

Additional reporting by Lillian Yang. With Associated Press and Reuters.