The Pentagon's Defense Intelligence Agency believes that the recent successes of the Armed Forces of Ukraine suggest a "realistic possibility" of a breakthrough in the rest of the Russian defence lines by the end of the year, although it would be extremely difficult.

After three months of slow progress, the Ukrainian counteroffensive is gaining momentum. "Had we had this conversation two weeks ago, I would have been slightly more pessimistic. Their breakthrough on that second defensive belt…is actually pretty considerable," Trent Maul, Director of Analysis at Defense Intelligence Agency (DIA), said in a rare interview with The Economist.

The DIA will monitor for signs that Russia may continue to supply artillery munitions to the front line and maintain leadership at the local level, Maul said.

Maul admits that American and Ukrainian officials failed to assess the depth of Russian defences and how difficult it would be for Ukraine to break through them with armoured vehicles. Ukrainian generals told the Guardian that 80% of Russia's efforts aimed to build the first and second lines of defence. But Maul warns that a significant part of Russian reinforcements remains on the third line.

Maul says two critical variables are extremely important: Ukraine's ammunition supply, vital to support the artillery that ensures the advance, and the weather in the fall.

One of the officials of the Biden administration says that Ukraine has about six to seven weeks of fighting left before the culmination of the offensive. Maul is somewhat less gloomy.

Maul said that Sergey Surovikin, the Russian general who built the defence lines, and Yevgeny Prigozhin, whose Wagner mercenaries have achieved Russia's most tangible successes in the past year, are both out of action, the former fired, and the latter killed in a plane crash.

Maul also says that Ukraine's recent successes are "significant" and give its forces a "realistic opportunity" – in intelligence terms, with a 40-50% probability – to break through the remaining Russian lines by the end of the year. But he warned that limited ammunition and deteriorating weather would make it "very difficult".

At the same time, the attention of the US intelligence community is already shifting to the next combat season. Even without a breakthrough this year, the DIA believes that if Ukraine can expand the salient around Robotyne, hold its ground and secure ammunition supplies, it will be well-prepared for another breakthrough in 2024.

NATO Secretary General Jens Stoltenberg said that Ukrainian troops were able to break through Russian defences and made progress in the counteroffensive.

US Secretary of State Antony Blinken said that Washington sees progress in the Ukrainian counteroffensive, and the US will ensure that Ukraine receives the necessary support both now and in the long term.