Washington CNN —  

President Barack Obama promised Friday to veto a plan by members of Congress to issue additional sanctions on Iran while diplomatic negotiations regarding Iran’s nuclear plan are under way.

“I will veto a bill that comes to my desk and I will make this argument to the American people as to why I’m doing so,” Obama said at a joint press conference with United Kingdom Prime Minister David Cameron, referencing a plan supported by some in his own party to increase sanctions on Iran through Congress before a deal is reached by international negotiators.

“My main message to Congress at this point is just hold your fire. Nobody around the world least of all the Iranians doubt my ability to get additional sanctions pass if these negotiations fail,” Obama added later.

Cameron said he was also calling American senators and urging them not to pass additional sanctions.

The wide ranging joint press conference covered issues as global as the terror attacks in France to as Beltway as what Obama thinks about his one time opponent Mitt Romney considering another presidential bid (“No comment,” Obama said while smiling).

Obama promised Friday to “do everything in our power” to assist France in their effort to combat terrorism in the wake of the attacks that killed at least 17 people across the country in the past week.

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“We will continue to do everything in our power to help France seek the justice that is needed and that all our countries are working together seamlessly to prevent these attacks,” Obama said.

On Iran, Obama turned a question on whether he’d veto additional sanctions on Iran back on his counterparts in Congress – including those in his own party.

“Why is it that we would have to take actions that would jeopardize the possibility of getting a nuclear deal over the next 60 or 90 days?” Obama asked.

Obama added later: “I am not, repeat not, suggesting that we are on immediate war footing should negotiations with Iran fail.”

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In a departure from the physical threats posed by those who attacked Paris, Cameron and Obama announced Friday new cooperation on combating cyberattacks, including cyber “war games” designed to identify vulnerabilities in banking networks.

Cameron is at the White House for bilateral talks expected to focus squarely on security after this month’s terror attacks in Paris and growing fears of violent Islamic terror cells inside Europe. Cameron and Obama addressed reporters in a joint press conference after their meeting Friday.

A British official said the two countries would establish “cyber cells” to share information and develop “a system where countries and hostile states and hostile organisations know that they shouldn’t attack us.”

The move comes after high profile breaches at Sony Pictures and the U.S. Central Command, ramping up concern about online safety.

British officials say Cameron flew to Washington with cyber issues at the front of mind. Cameron is worried that companies like Google, Facebook and Twitter are allowing terrorists to use their networks unseen by law enforcement. The companies say they have safeguards in place to ensure criminals and terrorists aren’t allowed to communicate.

Cameron told ITV in an interview that tech companies shouldn’t provide a “safe space” for terrorists to communicate or plan attacks.

The news conference will be the first time Obama is questioned about the Paris attacks, and his failure to attend a unity march held in the French capital last weekend. The White House says it was a mistake not to send a higher-profile administration official to the march.

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Jeremy Diamond contributed to this report.