WARSAW, Dec. 20 (Xinhua) -- Ukrainian President Volodymyr Zelensky said Friday that the situation on the battlefield is complicated and increasingly difficult.

Zelensky, who is on an official visit to Poland, told the media that he recently visited Kupyansk, a key transport hub in eastern Ukraine's Kharkiv region, depicting the situation there as challenging with the front line very close.

Despite pressure exerted by Russian forces, Zelensky said Ukrainian forces remain in control of the city.

Meanwhile, during his annual press conference on Friday, Russian President Vladimir Putin said that Kupyansk "came under our control several weeks ago," and that about 3,500 Ukrainian soldiers were encircled near the city.

Moreover, Zelensky said that there had been "problems with the availability of certain types of ammunition and air defense missiles for various reasons, and deliveries were also delayed." But he noted that this is not an accusation directed at Ukraine's partners. ■