KYIV, May 14 (Reuters) -

U.S. Secretary of State Antony Blinken told President Volodmyr Zelenskiy during a trip to Kyiv on Tuesday that part of a major U.S. aid package had arrived in Ukraine and that more was on its way that was going to "make a real difference".

Blinken's trip is the first by a senior U.S. official since Congress passed a long-delayed $61 billion aid package last month. Kyiv's outmanned troops are battling a new Russian offensive in the northeast as well as assaults in the east.

"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," Blinken said.

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

Ukraine recaptured swathes of territory during the first year after Russia's invasion in 2022, but a Ukrainian counter-offensive faltered last year and in recent months Russia has retaken the initiative at the frontline.

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.

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."

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.

The trip aimed to "send a strong signal of reassurance to the Ukrainians who are obviously in a very difficult moment," said a U.S. official who briefed reporters travelling with Blinken on condition of anonymity.

"The Secretary's mission here is really to talk about how our supplemental assistance is going to be executed in a fashion to help shore up their defences (and) enable them to increasingly take back the initiative on the battlefield," the official said.

Artillery, long-range missiles known as ATACMS and air defence interceptors approved by President Joe Biden on April 24 were already reaching the Ukrainian forces, the official said.

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.

security guaranteespeace summit, asking Blinken to help ensure participation from more countries. Moscow is not invited to the summit in Switzerland next month.

EXPANDING THE FIGHTING

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.

Russia's offensive into the north of Kharkiv region, home to Ukraine's second largest city of Kharkiv, continued on Tuesday. Ukraine's top military spy said he expected the situation to stabilise soon.

"They (the Russians) are clearly throwing everything they have in the east," said the U.S. official.

Economic and political reforms being undertaken by Kyiv will pave the way for the country to join the European Union and eventually NATO, the official said.

While the U.S.-led defence alliance is not likely to admit Ukraine any time soon, individual members are reaching bilateral security agreements with Kyiv. Talks on a U.S.-Ukraine agreement are "in the final stages" and will conclude ahead of a July NATO summit in Washington, the U.S. official said.

The Group of Seven wealthy nations signed a joint declaration at the NATO summit in Vilnius in July last year committing to establish "long-term security commitments and arrangements" with Ukraine that would be negotiated bilaterally.

Kyiv says the arrangements should contain important and concrete security commitments, but that the agreements would in no way replace its strategic goal of joining NATO. (Reporting by Simon Lewis; Writing by Humeyra Pamuk and Tom Balmforth; Editing by Don Durfee and Rosalba O'Brien)