KYIV, May 14 (Reuters) - The U.S. will stand by Ukraine until its security sovereignty is guaranteed, Secretary of State Antony Blinken pledged on Tuesday in a visit to Kyiv, at a time when Russia is mounting fresh attacks in the country's east.

The top U.S. diplomat is the first senior U.S. official to travel to Ukraine after U.S. Congress passed last month a $61 billion military aid package following a delay of several months during which Russia gained advantage on the battlefield.

In a policy speech at Igor Sikorsky Kyiv Polytechnic Institute, Blinken acknowledged the delay of U.S. weapons made Ukraine more vulnerable to Russia's attacks but said Kyiv can count on Washington's unwavering support because the American people understood that Russia will not stop in Ukraine.

"Putin is ramping up yet another offensive against Ukraine in Kharkiv and across the east, sending wave after wave of Russian soldiers, Iranian drones, North Korean artillery, and tanks, missiles and fighter jets built with machines and parts supplied by China," Blinken said.

"We are with you today. And we will stay by your side until Ukraine's security, sovereignty, its ability to choose its own path is guaranteed," he said.

Blinken arrived in Kyiv by train early on Tuesday morning on the previously undisclosed visit, which comes days after Russia launched a ground incursion into the north of the region of Kharkiv, opening a new front and stretching Ukraine's soldiers.

Kyiv has been on the back foot on the battlefield for months as Russian troops have slowly advanced, taking advantage of Ukraine's shortages of troop manpower and artillery shells.

Military aid from Washington, Kyiv's main backer, was held up for months, blocked by Republicans in the U.S. Congress until they finally allowed a vote last month, when it passed with support from both parties.

The hold up by the Republicans was largely driven by Donald Trump, former president and the party's presumptive nominee in November elections to rival President Joe Biden. His non-committal stance on Ukraine has spooked European countries.

In his meeting with Ukrainian President Volodymyr Zelenskiy, Blinken tried to reassure him that following the delays, U.S. aid would now be flowing in a steady way.

"We know this is a challenging time. But we also know that in the near term the assistance is now on the way, some of it has already arrived and more of it will be arriving," he said in remarks with Zelenskiy.

"And that's going to make a real difference against the ongoing Russian aggression on the battlefield."

Russia now controls about 18% of Ukraine and has been gaining ground since the failure of Kyiv's 2023 counter-offensive to make serious inroads against Russian troops dug in behind deep minefields.

Zelenskiy, addressing Blinken in English, said air defence supplies were "the biggest deficit for us" with Russia conducting long-range aerial attacks since March that have pounded electricity facilities and caused blackouts.

"Really we need today two Patriots for Kharkiv, for Kharkiv region because there the people are under attack. Civilians, warriors, everybody they are under Russian missiles."

On Monday, U.S. National Security Advisor Jake Sullivan said Washington was trying to accelerate "the tempo of the deliveries" of weapons to Ukraine to help it reverse its disadvantage.

"The delay put Ukraine in a hole and we're trying to help them dig out of that hole as rapidly as possible," Sullivan said, adding that a fresh package of weapons was going to be announced this week.

Zelenskiy also said he wanted to discuss security guarantees and an upcoming peace summit, asking Blinken to help ensure participation from more countries. Moscow is not invited to the summit in Switzerland next month.

Photo from Reuters