(CNN) - Speaking in Kiev, Sen. John McCain called Saturday for the United States to provide long-term military assistance to Ukraine, saying it is "the right and decent thing to do," as reports surfaced that Russian troops had traveled farther north into Ukraine from Crimea.
McCain was part of a bipartisan delegation of U.S. senators who traveled to Kiev ahead of Sunday's secession referendum in Crimea. The White House and U.S. allies in Europe have denounced the referendum as unconstitutional and illegal because Russian troops have essentially taken over the southern Ukraine peninsula.
McCain also had strong words for President Barack Obama in an op-ed appearing in The New York Times. He called for the President to take actions to restore the United States' credibility and strength around the world.
"Crimea has exposed the disturbing lack of realism that has characterized our foreign policy under President Obama. It is this worldview, or lack of one, that must change," wrote McCain.
Obama's administration, McCain wrote, supports the perception that the U.S. can "pull back from the world at little cost to our interests and values. This has fed a perception that the United States is weak, and to people like Mr. (Russian President Vladimir) Putin, weakness is provocative."
For McCain, this is embodied by a scaled-back missile defense program and defense budgets illustrating "hope, not strategy."
In his op-ed, McCain wrote that Iran and China have "bullied" America's allies and paid no price, and Bashar al-Assad crossed President Obama's "red line" by using chemical weapons in Syria and remains in power.
McCain portrayed Putin as determined to bring Russia's neighbors "back under Moscow's dominion by any means necessary." Putin's aggression in Crimea, McCain argued, is a symptom of "growing disregard for America's credibility in the world."
"Crimea must be the place where President Obama recognizes this reality and begins to restore the credibility of the United States as a world leader," McCain wrote.
In a press conference in Kiev, McCain and seven other senators, John Barasso (R-Wyo.), Richard J. Durbin (D-Ill.), Jeff Flake (R-Ariz.), John Hoeven (R-N.D.), Ron Johnson (R-Wis.), Christopher Murphy (D-Conn.) and Sheldon Whitehouse (D-R.I.), said Congress stood with the people of Ukraine. They called for strict sanctions against Russia. However, McCain went further than his colleagues.
"Ukraine is going to need a long-term military assistance program from the U.S. - equipment both lethal and nonlethal," said McCain.
"They ask for some modest means that can help them resist. I believe we should provide it," McCain said.
McCain's comments followed a report Thursday from the Wall Street Journal that the U.S. balked at a Ukrainian request for military aid, and was wary of heightening tensions with Russia.
When reached for comment by CNN, a senior administration official at the White House would only confirm that a number of requests from Ukraine are on the table. Ukrainian Prime Minister Arseniy Yatsenyuk was in Washington earlier in the week to meet with Obama and U.S. lawmakers.
McCain called on the Obama administration to come to the aid of the Ukrainian government and people "in their hour of greatest need."
McCain's trip to Ukraine followed his efforts over the past week to pass legislation in the Senate.
The Senate package includes $1 billion in U.S. loan guarantees, as well as $50 million to boost democracy building in Ukraine and $100 million for enhanced security cooperation for Ukraine and some of its neighbors.
It also includes proposed sanctions against individual Ukrainians and Russians responsible for the violence against anti-government protesters and those who have undermined the stability and sovereignty of Ukraine.
However, the measure has stalled in the Senate as it also includes approval of long-delayed reforms at the International Monetary Fund that are opposed by many Republicans.
McCain said on the Senate floor Thursday he was "embarrassed" that fellow Republicans were putting disputes over the IMF and campaign finance reform ahead of the Ukrainians.
But in Kiev on Saturday, McCain sounded more optimistic. He said that Congress and Obama would work together on supporting Ukraine.
"This is too serious for partisanship," McCain said.
Murphy, a supporter of the measure, also spoke of the Senate taking an active role as events unfold in Crimea.
"The world is watching, and the message that we bring today both to Ukraine and to Russia is that the United States Senate is watching as well," he said.
McCain and Durbin said they spoke with Secretary of State John Kerry and Samantha Power, U.S. Ambassador to the United Nations. Kerry returned to Washington after meeting with Russia's foreign minister in London.
Power was present at the U.N. Security Council in New York, where a U.S. drafted resolution declaring the Crimea referendum invalid was vetoed by Russia. Thirteen of the 15 Security Council members backed the resolution, while China abstained from the vote.
Overshadowing the events of the day were reports from the State Border Guard Service of Ukraine that about 60 Russian troops in six helicopters assisted by three armed vehicles crossed into Ukraine's Kherson region, which borders Crimea.
The State Border Guard Service of Ukraine said the Russian troops were on the ground and the Ukrainian guards had taken defensive positions.
Power, speaking to press after the session, said that if Russian troops had crossed further into Ukraine it would be an "outrageous escalation."