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7 year oldGeneral James Mattis, defence secretary nominee, warned Nato was under its biggest attack since World War Two.
Mike Pompeo, Mr Trump's pick to lead the CIA, said Moscow posed a threat in Europe and was "asserting itself aggressively" in Ukraine.
The tough talk follows Mr Trump's call for warmer relations with Moscow.
Mr Mattis, a retired general and Mr Trump's pick for Pentagon chief, said Russian President Vladimir Putin was trying to divide Nato nations.
"I think right now the most important thing is that we recognise the reality of what we deal with with Mr Putin," he told the Armed Services Committee.
"And we recognise that he is trying to break the North Atlantic Alliance and that we take the steps... to defend ourselves where we must.
"I think it's under the biggest attack since World War II, sir, and that's from Russia, from terrorist groups and with what China is doing in the South China Sea."
If confirmed, Mr Mattis, nicknamed Mad Dog, would be the first career military officer to serve as secretary of defence in more than 50 years.
But the committee would have to approve an exception to a rule that says former military staff must spend a minimum of seven years out of office before they can run the Pentagon.
Mr Pompeo, Mr Trump's choice for CIA director, faced questions over the president-elect's rift with the US intelligence community.
He told the Intelligence Committee he had "every confidence" in the US intelligence services.
Their assessment that state-directed Russia hackers meddled in the US election was "sound", he added.
The conservative Kansas Republican was quick to criticise Russia for invading and occupying Ukraine, threatening Europe and "doing nearly nothing" to destroy the so-called Islamic State.
But when asked what was the greatest security threat to the US, he cited terrorism foremost and lumped Russia in behind North Korea and China.
Mr Pompeo, who previously said the CIA's post-9/11 interrogation practices were "within the law", was questioned about that issue, too.
He was asked if he would support the statement that "the CIA is out of the enhanced interrogation" business.
Mr Pompeo said: "Yes, you have my full commitment to that."
He added he would "absolutely not" restart those programmes if asked to do so by the president-elect.
Rex Tillerson, Mr Trump's choice for secretary of state, is also undergoing a second day of grilling before the Foreign Relations Committee.
The former chief executive of Exxon Mobil faced sharp questioning on Wednesday from Republican senator Marco Rubio.
Ben Carson, Mr Trump's nominee for housing secretary, is also being questioned by senators.
Mr Carson, who ran for president, is a retired neurosurgeon with no direct experience of housing policy or of public office in general.
He was raised in a low-income Detroit neighbourhood by a mother who worked several jobs and relied on welfare to support her sons.
But he has heavily criticised welfare programmes saying that they breed dependency.
<p>A US judge has ruled against Donald Trump getting his hush money conviction thrown out on immunity grounds.</p>