After more than 15 years in Afghanistan the US still does not have a strategy for winning peace and is making that goal even more unattainable by hampering diplomacy, a bipartisan group of US senators said in the Afghan capital on Tuesday.

The criticism came as the Trump administration considers the deployment of thousands of additional soldiers, without publicly explaining what they are meant to achieve.

In Kabul, the Republican senator John McCain excoriated 15 years of US efforts in Afghanistan, which, he said, pursued a goal amounting to “don’t lose”, rather than winning.

McCain – accompanied by the Democratic senators Elizabeth Warren and Sheldon Whitehouse, and fellow Republicans Lindsey Graham and David Perdue – said the delegation shared concerns about the worsened security in Afghanistan since the drawdown of coalition troops in 2014.

“Each of us may describe that concern in our own way but none of us would say that we’re on course to a success here in Afghanistan,” he said.

Warren added that without a clear plan, political patience in the US could run out. “We need a strategy in the United States that defines our role in Afghanistan, defines our objective and explains how we’re going to get from here to there,” she said.

Most experts believe the Afghan conflict cannot be won by military means alone. Yet, while planning to boost the 8,400 US troops in Afghanistan with about 4,000 more, Donald Trump is gutting civilian bodies tasked with diplomacy.

Proposing budget cuts of 32% to the state department, Trump has also allowed for the closure of the office of the special representative to Afghanistan and Pakistan, tasked with coordinating across the US government to meet strategic goals in the region.

“I see a lack of focus that’s very unnerving,” said Graham, calling the state department “woefully understaffed”.

Whitehouse agreed, commending the military for “pushing back at the hollowing-out of the state department in the early months of this administration”.

The congressional delegation visited Afghanistan and Pakistan before a US review of its Afghan war strategy.

“The strongest nation on Earth should be able to win this conflict,” McCain said. “And we are frustrated that this strategy hadn’t been articulated yet, to be honest with you.”

In Islamabad on Monday, McCain said Pakistan’s cooperation was crucial to peace in the region.

The US government has already shown signs of a hardened line against Pakistan, which experts believe has sheltered and supported militants from the Afghan Taliban for years, and not sufficiently curbed terrorist groups.

Last week, the US designated Syed Salahuddin, a Kashmiri militant leader, as a terrorist, prompting accusations from Pakistan that Trump was doing the bidding of India’s prime minister, Narendra Modi, who was visiting Washington at the time.

Also last week, Gen Joseph Dunford, the chair of the joint chiefs of staff, visited Kabul for a final evaluation of whether to increase US troop levels.

When asked what winning in Afghanistan would look like, McCain stopped short of demanding a military defeat of the Taliban, settling for “an advantage on the battlefield”.

“Winning is getting major areas of the country under control, and working towards some kind of ceasefire with the Taliban,” he said. “They will not negotiate unless they think they are losing.”

Graham said American soldiers were in Afghanistan to protect the US.

“Leaving radical Islam alone will not make us safe at home,” he said. “I want every American to know that we will win this thing because the Afghan people do not want to go back to the darkness. They want to pursue the light.”