US Secretary of State Antony Blinken said that while Ukraine has recaptured half the territory that Russia initially seized in its invasion, Kyiv faced "a very hard fight" to win back more.
"It's already taken back about 50 per cent of what was initially seized," Mr Blinken said in an interview with CNN on Sunday.
"These are still relatively early days of the counteroffensive. It is tough," he said, adding: "It will not play out over the next week or two. We're still looking I think at several months."
Late last month, president Volodymyr Zelenskiy was quoted as saying that the counteroffensive's progress against Russian forces was "slower than desired".
Ukraine has recaptured some villages in the south and territory around the ruined city of Bakhmut in the east, but has not had a major breakthrough against heavily defended Russian lines.
When asked if Ukraine will get US-made F-16 fighter jets, Mr Blinken said he believed it would. "And the important focus is on making sure that when they do, they’re properly trained, they’re able to maintain the planes, and use them in a smart way."
A coalition of 11 nations will start training Ukrainian pilots to fly the F-16 fighter jets in August in Denmark, and a training centre will be set up in Romania.
Ukraine has long appealed for the Lockheed Martin-made F-16s, but US national security adviser Jake Sullivan, said last month there was no final decision on Washington sending the aircraft. US officials have estimated it would take at least 18 months for training and delivery of the planes.
The United States has given Ukraine more than $41 billion in military aid since Russia invaded in February 2022.
