KHARKIV, Ukraine (Reuters) - Thousands took to the streets of Ukraine's second-largest city on Saturday carrying banners saying "Kharkiv is Ukraine" and "stop Russian aggression", as the country braced for a possible military offensive from Russia.

       Weeks of diplomacy between the West and Moscow have produced no breakthrough after Russia massed tens of thousands of troops near Ukraine's borders.

       Moscow denies it plans to attack Ukraine but has demanded security guarantees, including a block on Ukraine joining the NATO alliance.

       Kharkiv, an eastern industrial city that lies 42 km (26 miles) from the Russian border, was identified by Ukrainian President Volodymyr Zelenskiy as a possible Russian target, though his spokesman later said he was speaking hypothetically.

       Demonstrators in Kharkiv sang the national anthem and waved Ukrainian flags, or held up flags of allies that have supported Kyiv, including the United States, Britain and the European Union.

       "The people came into the streets to demonstrate that Kharkiv is a Ukrainian city and we will not surrender it," said Kharkiv resident Nina Kvitko.

       (Editing by Matthias Williams and David Holmes)