If NATO partners eventually cease to maintain attack aircraft capable of delivering fielded U.S. nuclear bombs, then allied jets could "pick up the load."

Air Force Chief of Staff Gen. Mark Welsh raised that possibility during a congressional hearing earlier this month when asked about contingency planning for a potential future in which some European nations that currently host U.S. nuclear weapons opt to retire -- and not replace -- today's aircraft that are capable of carrying either nuclear or conventional munitions.

Five NATO countries -- Belgium, Germany, Italy, the Netherlands and Turkey -- are understood to host a net total of fewer than 200 B-61 gravity bombs, though the United States does not formally acknowledge nuclear-basing details.

"As NATO nations -- if they choose not to upgrade their own nuclear aircraft capabilities, then other NATO nations that have those capabilities from an operational perspective will pick up the load," Welsh said during a March 14 appearance before the House Armed Services Committee. "That'll be a NATO policy decision. The U.S. will be part of that discussion. We do have the capacity to pick up the load."

In his remarks, Welsh did not definitively make clear whether the U.S. Air Force or other alliance members would take on the additional aircraft mission responsibility.

Still, the general's comments suggest the Pentagon is planning for its tactical nuclear weapons role in Europe to continue, irrespective of the future air-delivery capability of NATO hosting states. Arms control advocates had previously argued that the United States should withdraw its nonstrategic weapons from the continent if NATO partners do not modernize their dual-capable aircraft.

The Obama administration's 2013 unclassified report to Congress on nuclear-employment guidance states that the U.S. military would "maintain the capability to forward-deploy nuclear weapons with heavy bombers and dual-capable aircraft in support of extended deterrence and assurance of U.S. allies and partners."

Nuclear weapons continue to be a "core component" of NATO's deterrence against aggression in Europe, the alliance stated in its 2012 Deterrence and Defense Posture Review. At the same time, the allies also said they were prepared to consider reductions to the current number of tactical atomic arms assigned to the defense of NATO nations.

Defense Department spokeswoman Cynthia Smith in an e-mail to Global Security Newswire said Welsh's comments were in line with these U.S. and NATO policies.

The Pentagon and Air Force did not respond to separate requests for comment on whether any other countries besides the United States were being considered for possibly taking on a new role in the NATO B-61 air delivery mission.

However, according to issue expert Hans Kristensen, the United States is the only NATO country with the current military capacity to handle the extra burden.

The Air Force has nuclear-capable jets based in Italy, Germany and the United Kingdom that could take on a larger share of the nuclear bombs, said Kristensen, who closely monitors developments in the NATO atomic mission.

"The U.S. certainly has the capacity in its Air Force to pick up the slack," Kristensen, who directs the Federation of American Scientists' Nuclear Information Project, said in a recent phone interview.

At the core of the issue is aging aircraft. All five host nations of the nuclear bombs field either dual-capable F-16 or Tornado strike aircraft slated for retirement in the 2020s.

Some of the countries have said the attack-plane replacements they plan on purchasing would be dual capable, while others have hinted they would allow the nuclear-delivery role to expire along with the aircraft retirements.

Most of the current hosting nations are signed up or in talks to acquire the F-35 Joint Strike Fighter, which is intended to include a future version capable of carrying the B-61 nuclear bomb.

"There is one overall trend, which is none of them can afford as much as they wanted," Kristensen said of the NATO partner states' ability to buy the new aircraft.

The Dutch government in January confirmed that some of the Joint Strike Fighters it plans to purchase could have a nuclear role, ignoring a 2012 resolution by its parliament urging that the jets not have a dual capability.

"The Belgians will probably follow the Dutch in whatever they do," in terms of deciding whether to buy new multirole aircraft, Kristensen said. He noted that the Belgian parliament had passed a resolution calling for the withdrawal of U.S. nuclear weapons from the country.

Belgium reportedly is in talks to also purchase the Lockheed Martin-produced F-35 to replace its aging fleet of F-16 jets.

Turkey is planning on replacing its F-16 fighters with F-35s. Some of those new jets are expected to be dual-capable, so that Ankara can maintain its role in NATO's nuclear-deterrence mission, according to Aaron Stein, an associate fellow at the Royal United Services Institute in London.

Italy also is planning to acquire the Joint Strike Fighter, though ongoing budget cuts could mean that Rome reduces its current order of 90 planes.

The Italians have been cutting back their JSF plans for several years -- not just in terms of unit quantity, but also in the amount of training hours their pilots will get on the planes, according to Kristensen.

Of all the NATO nuclear-weapons hosts, Germany has given the strongest indications it will allow its participation in the role to eventually lapse. Berlin is replacing its dual-capable Tornado aircraft with the Eurofighter Typhoon, which is not designed to carry the B-61 bomb.

The German government already has extended the service life of its Tornados until the 2025-to-2030 time frame, Kristensen said.

"Beyond that, it begins to get shaky, because aircraft only fly for so long," he said.

"It is up to each ally to decide what military capabilities they acquire or retain. This includes aircraft which can carry nuclear weapons," a NATO official based at alliance headquarters in Brussels said in a written statement. "We would expect allies who contribute to NATO's nuclear sharing arrangements to inform allies should their contribution change."

The official provided the comments to GSN on condition of not being named.

Some NATO member states in Central and Eastern Europe favor continued deployment of the gravity bombs as a signal to Russia, but it remains unclear how that might affect which nations play a role in the mission. The NATO pro-nuclear contingent is seen to have gotten a boost following Russia's recent annexation of Crimea, which has prompted new concerns about potential further incursions into former Soviet or Warsaw Pact states.

However, Steven Pifer, head of the Brookings Institution's Arms Control and Non-Proliferation Initiative, said there is little chance of these nations taking on the tactical-bomb delivery role.

"I think from NATO's perspective, moving nuclear weapons into a Central European country would be seen as provocative [toward Russia], but also militarily more vulnerable," the onetime U.S. ambassador to Ukraine said to GSN.

If any NATO countries from Eastern Europe were interested in hosting U.S. nuclear bombs, there would be "enormous political obstacles" standing in the way, Kristensen agreed.

NATO leaders know it would be destabilizing to shift deployed U.S. tactical warheads eastward, perhaps even more so in the context of the current sky-high tensions with Russia over its incursion in Ukraine, Pifer suggested.

"I've actually heard a central European representative say half-jokingly, if the German's don’t want them, we'll take them," he said.

Moreover, "none of the countries in the western part of NATO would touch this with a 10-foot pole," said Kristensen.

The matter of continuing to deploy U.S. tactical weapons in Europe at all remains considerably controversial.

A growing view in Belgium, Germany and the Netherlands is that the nuclear bombs serve little military value and should be withdrawn. Some argue the nuclear arsenals based in France, the United Kingdom and the United States are sufficient for providing deterrence for the entire alliance.

This all makes Washington the most likely NATO member to take up the additional nuclear-delivery responsibility, analysts agreed.

The U.S. military already has fighter wings in Europe with the capability of delivering the B-61 bomb. Nuclear-capable U.S. jets that could be given the mission include F-16 aircraft based at Aviano Air Base in Italy, F-15E jets at Royal Air Force Lakenheath based in the United Kingdom, and F-16s fielded at Spangdahlem Air Base in Germany, according to recent data compiled by Kristensen and fellow FAS nuclear analyst Robert Norris.

The U.S. multirole planes deployed in Europe do not presently have B-61 bombs assigned to them, but that could change, according to the nuclear forces experts.

"It wouldn't be that the U.S. would have to add a wing," Kristensen said. "It could just continue with the wings it already has."