By Anastasiia Malenko and Aleksandar Vasovic

KYIV (Reuters) -Russia has made incursions near two towns key to army supply routes in eastern Ukraine, a Ukrainian military official said on Wednesday, as Moscow seeks a breakthrough in a summer offensive at a time of uncertainty over U.S. support for Kyiv.

In recent weeks, Russia has amassed forces and despite heavy losses has advanced in rural areas either side of Pokrovsk and Kostiantynivka, which both sit on crossroads running to the frontline from larger cities in Ukrainian-controlled territory.

Russia's advances on the front are matched by an intensification of drone and missile strikes on Kyiv and other cities, following signs that Washington's support for Ukraine's war effort is faltering.

So far, the efforts by U.S. President Donald Trump have failed to achieve a ceasefire in the full-scale invasion launched by Russia in 2022.

One of the aims of the Russian offensive is to occupy the rest of the Donetsk region. Now, they are using small assault groups, light vehicles, and drones to push towards the neighbouring region, said Viktor Trehubov, a spokesperson for the Khortytsia group of forces.

"There are constant attacks with the intent of breaking through" to the border of the Dnipropetrovsk region at any cost, Trehubov said in written comments to Reuters.

Russia now has 111,000 soldiers in the Pokrovsk area, which it has been trying to seize since early last year, Ukraine's top armed forces commander Oleksandr Syrskyi said last week, describing dozens of battles in the area every day.

A decision by Washington to halt some deliveries of various weapons including precision rocket artillery to Kyiv will worsen the situation on the ground for Ukraine's forces, said Jack Watling, a senior researcher at the Royal United Services Institute, a think-tank.

"The loss of these supplies will significantly degrade Ukraine's ability to strike Russian forces beyond 30 km (19 miles) from the front line and therefore allow Russia to improve its logistics," Watling said.

RUSSIAN GAINS

Ukrainian blog DeepState, which uses open-source data to map the frontline, said the Russian military in June had seized 556 square kilometres of Ukrainian territory, which it said was the largest monthly loss of ground since November.

Russian forces, which have numerical superiority, cut the main road linking Pokrovsk and Kostiantynivka in May, complicating Ukrainian movements and resupply efforts.

"The Russian advance is being contained, but their crossing of the Pokrovsk-Kostyantynivka highway is a strategic and logistical setback," Trehubov said.

Heavy Russian losses have prevented Russian advances toward Kostiantynivka via Chasiv Yar, or along the western Pokrovsk front. "Now they are attempting (to advance) further away from populated areas," Trehubov said.

DeepState also reported that Russian advances in June near Pokrovsk and nearby Novopavlivka accounted for more than half of all Russian gains along the entire frontline in all of Ukraine.

Trehubov said Pokrovsk and Kostyantynivka remain Ukrainian logistical hubs, despite setbacks and drone activity which make some defensive fortifications less effective.

"(Drones) hinder logistics for both sides but don't make it impossible. Drones after all are not invulnerable," he said.

(Reporting by Anastasiia Malenko and Aleksandar Vasovic; Additional reporting by Tom Balmforth; Editing by Frank Jack Daniel and Gareth Jones)