The trip comes at a time when tensions between Moscow and the West are at their highest following Ukraine's strikes on US and British missiles on Russian territory and Russia's launch of an experimental hypersonic missile and nuclear threats.
The European Union's new leadership is keen to show firm support for Ukraine as its forces retreat to the front line and with Donald Trump's imminent arrival in the White House, raising fears of a reduction in US aid to Kiev.
"We came to send a clear message: we stand with Ukraine and we continue to give it our full support," Costa told accompanying reporters.
In addition to the difficulties on the front, Ukraine has been subjected to several waves of strikes targeting its energy infrastructure in recent weeks, which have led to massive power outages as winter approaches.
"The situation in Ukraine is very, very serious, but it is clear that it also has a very high cost for Russia," said Kallas, a former Estonian prime minister who is known for his tough stance on Moscow.
The two officials, who occupy the EU's top posts after European Commission President Ursula von der Leyen, are due to hold talks with Ukrainian President Volodymyr Zelensky. | BGNES
