WASHINGTON (Reuters) - Only six more nations need to ratify an amendment to a nuclear security convention that would make it legally binding for countries to tighten protection of nuclear facilities and materials, the head of the International Atomic Energy Agency said on Wednesday. IAEA Director Yukiya Amano told reporters that the measure  should take effect “in the near future” after Serbia and the Marshall Islands formally ratified it on Wednesday and more countries were expected to submit paperwork on Thursday or Friday. It was not immediately clear which countries they were. Amano, in Washington for President Barack Obama’s fourth and final Nuclear Security Summit this week, said the ratification would be “a very important step up ... in nuclear security.” Pakistan ratified the amendment last week. “Entry into force would reduce the likelihood of terrorists being able to detonate a radioactive dispersal device, otherwise known as a ‘dirty bomb,’” Amano said in a speech in Washington earlier Wednesday. Amano has pressed hard to advance the amendment to the Convention on the Physical Protection of Nuclear Material, which was adopted by 152 member countries a decade ago. Two thirds of the members, or 102 countries, must ratify the long-delayed measure for it to take effect. The amendment makes it legally binding for countries to protect nuclear facilities, as well as the domestic use, storage and transportation of nuclear material. It provides for expanded cooperation among countries on finding and recovering stolen or smuggled nuclear material. States would be required to minimize any radiological consequences of sabotage, and to prevent and combat any such offenses. Amano said that more work was needed in making the amendment   universal, which would help ensure that all countries with nuclear capabilities - including North Korea - adhered to the measure, not just those countries that had ratified the measure. The IAEA director said he favored organizing a separate conference to review the amendment once it took effect.   