Visiting Secretary General of the North Atlantic Treaty Organization (NATO) Jens Stoltenberg on Tuesday stressed diplomatic efforts to solve the ongoing Ukraine crisis.

Stoltenberg made the remarks at a joint press conference with Estonian Prime Minister Kaja Kallas and visiting British Prime Minister Boris Johnson at the Tapa Army Base in northern Estonia.

He called for an immediate stop of the war in Ukraine, pullout of Russian forces and the engaging of diplomatic efforts.

"Over the last weeks, in response to Russia's attacks on Ukraine, we have increased our defensive presence in the air, on land, and at sea," he noted.

There are "over 100 jets at high alert operating from 30 different locations and over 120 ships from the Baltic Sea to the Mediterranean," Stoltenberg said, adding that Britain, the United States and other allies are deploying thousands more troops to the eastern part of the alliance.

"For the first time in our history, we are deploying the NATO response force," said the NATO chief, who termed NATO as a defensive alliance that does not seek conflict with Russia.

The second round of peace talks between Russia and Ukraine could take place on Wednesday, the TASS news agency reported Tuesday citing sources.

The first round of the negotiations, which lasted about five hours, concluded on Monday in Belarus' Gomel region, with no clear breakthrough.

For his part, Johnson said that he wanted to be "crystal clear" that "we will not fight Russian forces in Ukraine" and "our reinforcements, like the reinforcements here in Tapa, are firmly within the borders of NATO members."

On the number of reinforced troops, the British prime minister said "we will always keep things under review." He also termed the "no-fly zone" on Russian aircraft over Ukraine as a very big step that "simply is not on the agenda of any NATO country."

More than 900 members of the Royal Welsh infantry regiment of the British Army and nearly 200 Danish troops together with their vehicles and equipment will be joining NATO Battlegroup Estonia at Tapa Army Base, the Estonian Public Broadcasting reported on Tuesday.

Estonian President Alar Karis met with Stoltenberg at Tallinn airport earlier on the day.

Stoltenberg also made a trip to Poland with a visit to Lask airbase on Tuesday.